feat(files): per-stack file explorer (#780)

* feat(files): backend foundation for stack file explorer

Install multer for multipart file upload handling. Add
isValidRelativeStackPath to validation.ts to guard client-supplied
relative paths against traversal, absolute paths, NUL bytes, backslash
injection, and double-slash segments. Add isBinaryBuffer to a new
binaryDetect.ts utility for heuristic text/binary detection via
NUL-byte fast exit and non-printable byte ratio sampling.

* fix(files): reject bare dot segments in isValidRelativeStackPath

* feat(files): add safe stack-scoped file I/O methods to FileSystemService

Adds FileEntry interface and seven new public methods to FileSystemService
for stack-scoped file operations: listStackDirectory, readStackFile,
streamStackFile, writeStackFile, deleteStackPath, mkdirStackPath, and
statStackEntry.

Each method routes through a private resolveSafeStackPath helper that
enforces two-phase path containment: a pre-realpath lexical check plus a
post-realpath symlink-escape check. ENOENT targets are handled by walking
up to the deepest existing ancestor, realpaths that ancestor, and
reattaching the remaining suffix.

Binary detection delegates to isBinaryBuffer; path safety delegates to
isPathWithinBase. Protected file names and the MIME map are module-level
constants to avoid repeated allocation.

* feat(files): frontend API wrappers and Monaco language helper

* fix(files): tighten stackFilesApi error handling and localOnly support

* fix(files): FileSystemService safety and correctness fixes

* feat(files): add file explorer API endpoints to stacks router

* feat(files): FileTree and FileTreeNode components

* fix(files): route security hardening and stream cleanup

* fix(files): FileTree accessibility, icon stroke, stale fetch guard

Add strokeWidth={1.5} to all Lucide icons in FileTreeNode to match the
design system. Add aria-expanded to directory rows for accessibility.
Guard handleDirClick .then() callbacks against stale stack name
references when the component re-renders with a new stack. Add
toast.info fallbacks when compose.yaml or .env is clicked without a
navigation callback registered.

* feat(files): FileViewer, FileUploadDropzone, NewFolderDialog, DeleteFileConfirm

* fix(files): resolve code quality findings in file explorer components

- Move editorOptions useMemo above conditional returns in FileViewer (Rules of Hooks fix)
- Fix blob download: append anchor to DOM before click, defer URL revoke 100ms
- Keep protected-file confirm input visible during NOT_EMPTY recursive retry in DeleteFileConfirm
- Remove non-functional cursor-pointer/onClick from Community upgrade pill in FileUploadDropzone
- Add success toast on folder creation in NewFolderDialog
- Switch all (e as Error).message casts to instanceof Error narrowing

* test(files): unit tests for binary detection, stack path safety, and file explorer routes

- binary-detection.test.ts: covers isBinaryBuffer edge cases (empty, NUL,
  PNG header, threshold boundary, sampleBytes parameter)
- filesystem-stack-paths.test.ts: covers isValidRelativeStackPath (accepts/
  rejects matrix) and FileSystemService stack methods against a real temp dir
  (listStackDirectory sort and protection flags, readStackFile text/binary/
  oversized paths, writeStackFile/Buffer, deleteStackPath, mkdirStackPath,
  traversal guard); platform-specific empty-dir/NOT_EMPTY cases skip on Windows
- stack-files-routes.test.ts: route-level integration tests for all seven
  file explorer endpoints; covers auth gating, Community-tier 403 gates,
  input validation, 413 TOO_LARGE upload limit, and 204/200 happy paths

* feat(files): StackFileExplorer container with lazy tree, viewer, and action bar

* fix(files): add Download button to explorer toolbar, fix Community upgrade pill, reset state on stack change

* test(files): add missing test coverage for file explorer routes and service

* feat(files): add Files tab to EditorLayout with StackFileExplorer integration

* fix(files): add defensive activeTab guard to saveFile and discardChanges

* test(files): unit tests for FileTree expand/collapse and FileViewer render modes

Covers the three FileViewer content modes (text/Monaco, binary panel,
oversized panel) and the FileTree expand/collapse/cache cycle: first
expand fetches the subdirectory, second click collapses without a fetch,
third click re-expands from the in-memory cache without a second fetch.

* test(e2e): file explorer community and skipper+ flows

Covers the full file-explorer feature surface in two describe blocks:

Community (read-only): intercepts /api/license to simulate community
tier, confirms the upgrade pill is visible in the left pane, and
asserts that the Save button is absent after opening a text file.

Skipper+ (full CRUD): uploads a text file and confirms it appears in
the tree; edits config/app.conf and saves via Monaco; deletes an
uploaded file and asserts the tree entry is gone; issues a raw HTTP
request to the download endpoint and checks for status 200 and the
content-disposition: attachment header.

Also adds data-testid="file-action-delete" to the action bar Delete
button in StackFileExplorer for stable targeting, and exports
waitForStacksLoaded from e2e/helpers.ts to eliminate the three
identical local copies in stacks, deploy-log-panel, and stack-files
spec files.

* fix(e2e): improve test isolation and selector stability in stack-files spec

Move beforeEach seed to beforeAll/afterAll so fixtures are created once per
suite, not before every test. Extract shared seedSuite/teardownSuite helpers
to eliminate the duplicate beforeAll/afterAll blocks. Wrap teardown in
try/catch so failures log a warning rather than masking test results.

Replace waitForTimeout(500) with a deterministic expect on the file tree
sentinel. Add data-testid="anatomy-files-btn" and data-testid="delete-confirm-btn"
to replace the fragile button text/positional selectors. Assert Save button
starts disabled before editing.

* docs(files): add stack file explorer documentation

Add user-facing guide for the stack file explorer feature covering
tier access (Community read-only, Skipper+ read-write), viewing
limits, upload/download caps, protected file routing, and
troubleshooting. Update the editor page to reference the new guide
and register the page in the navigation.

* fix(docs): use canonical Skipper tier name in file explorer overview card

* fix(files): resolve lint errors blocking CI

Remove unnecessary backslash escape before double-quote in the
Content-Disposition regex (no-useless-escape). Replace five synchronous
setState resets at the top of the FileTree mount effect with a React key
prop on the FileTree element in StackFileExplorer so remounting resets
state automatically, eliminating the react-hooks/set-state-in-effect
violation.

* test(files): fix e2e seeding to work on community-tier CI

Replace the browser-side paid upload/mkdir API calls in seedTestStack with
direct Node fs writes. The upload and folder endpoints require Skipper+ so
they returned 403 on CI, which runs with no license set. Stack creation
via POST /api/stacks stays as an API call since it is community-allowed and
keeps the backend registry in sync.

Add a per-test tier check in the Skipper+ beforeEach that skips gracefully
when the instance is community, matching the pattern in auto-heal-policies.

frontend/src/components/files/FileTreeNode.tsx

@@ -0,0 +1,60 @@
+import { ChevronRight, ChevronDown, Folder, File, Link, Loader2 } from 'lucide-react';
+import type { FileEntry } from '@/lib/stackFilesApi';
+import { cn } from '@/lib/utils';
+
+interface FileTreeNodeProps {
+  entry: FileEntry;
+  depth: number;
+  isSelected: boolean;
+  isExpanded?: boolean;
+  isLoading?: boolean;
+  onClick: () => void;
+}
+
+export function FileTreeNode({
+  entry,
+  depth,
+  isSelected,
+  isExpanded,
+  isLoading,
+  onClick,
+}: FileTreeNodeProps) {
+  const isDir = entry.type === 'directory';
+
+  return (
+    <div
+      role="button"
+      tabIndex={0}
+      onClick={onClick}
+      onKeyDown={(e) => {
+        if (e.key === 'Enter' || e.key === ' ') onClick();
+      }}
+      aria-expanded={isDir ? isExpanded : undefined}
+      className={cn(
+        'flex items-center gap-1.5 py-0.5 cursor-pointer select-none rounded-sm',
+        isSelected
+          ? 'bg-accent text-accent-foreground'
+          : 'hover:bg-accent/50 text-foreground'
+      )}
+      style={{ paddingLeft: depth * 16 + 8 }}
+    >
+      {isDir && (
+        isLoading
+          ? <Loader2 className="w-3.5 h-3.5 shrink-0 animate-spin" strokeWidth={1.5} />
+          : isExpanded
+            ? <ChevronDown className="w-3.5 h-3.5 shrink-0" strokeWidth={1.5} />
+            : <ChevronRight className="w-3.5 h-3.5 shrink-0" strokeWidth={1.5} />
+      )}
+      {isDir
+        ? <Folder className="w-3.5 h-3.5 shrink-0" strokeWidth={1.5} />
+        : entry.type === 'symlink'
+          ? <Link className="w-3.5 h-3.5 shrink-0" strokeWidth={1.5} />
+          : <File className="w-3.5 h-3.5 shrink-0" strokeWidth={1.5} />
+      }
+      <span className="font-mono text-sm truncate">{entry.name}</span>
+      {entry.isProtected && (
+        <span className="w-1.5 h-1.5 rounded-full bg-amber-500 shrink-0" />
+      )}
+    </div>
+  );
+}