feat(stacks): browse and edit mounted volume files in the explorer (#1403)

* feat(stacks): browse and edit mounted volume files in the explorer

Reposition the stack file explorer around runtime configuration access:
discover a stack's declared mounts and expose each as a safe, stack-scoped
file root. The explorer opens on a Volumes group (bind mounts and named
Docker volumes) by default, with the stack source directory as a secondary
group, on a "Files & Volumes" tab.

- Discover roots from the rendered effective compose model; resolve named
  volumes to their Docker name and browse/edit them through the hardened
  helper container, with bind mounts handled directly when reachable.
- Re-derive the allowed roots server-side on every file operation and match
  the client root id against them, so a request can never address a path the
  stack did not declare. Block dangerous host mounts and binds that overlap
  Sencho's managed directories; reject writes to read-only mounts.
- Thread an optional root id through the existing file endpoints and an
  opaque, parseable optimistic-concurrency token through read, conflict,
  and write, for both filesystem and helper backends.
- Keep compose and env file protection on the stack source root only.

* fix(stacks): theme the Files & Volumes root switcher

Replace the raw native select in the file-root switcher with the design
system Select component. The native control did not honour the dark theme,
so the panel rendered white with unreadable text. The themed Select gives a
dark popover with grouped Volumes / Stack source labels and disabled items.

* fix(stacks): contain the bind-root probe and de-taint the file-op error log

Gate the volume-root bind probe's realpath/stat behind a compose-base
containment check (mirroring the storage host-path probe) so they never run
on an unvalidated host path; a source outside the compose dir is unreachable
in the containerized deployment anyway and is reported non-accessible without
touching the filesystem. Log the helper-backed file-op failure through a
constant format string with sanitized arguments instead of an interpolated
template literal.

* fix(stacks): inline the bind-probe containment guard at the fs sinks

The wrapped containment predicate was not recognized as a path barrier, so
the bind probe's realpath/stat still flagged as uncontrolled-data-in-path.
Inline the path.resolve + startsWith check directly at each filesystem sink
(and re-check the resolved canonical before stat, so a within-base symlink
that resolves outside the compose dir is also rejected).

* fix(stacks): harden file-root lifecycle, upload race, and helper errors

Address review findings on the Files & Volumes feature:

- Invalidate the file-root allowlist on stack create/delete/import/from-git
  (wire StackFileRootsService.invalidateNode into invalidateNodeCaches), so a
  stack deleted and recreated under the same name cannot serve the old stack's
  roots from the 15s cache.
- Use the atomic exclusive write for a non-overwrite upload so a file created
  by another writer after the existence check is not silently clobbered.
- Let the helper's real cd errno through and map permission failures to 403
  consistently across list/stat/read/write/mkdir/delete/pathKind, instead of
  reporting EACCES as 404/500; pathKind no longer reports a permission-denied
  parent as absent.
- Document the realpath-then-open TOCTOU as a known, pre-existing limitation of
  every file op (O_NOFOLLOW is not viable because config volumes legitimately
  contain symlinks); the bind root is contained to the compose dir and the op
  requires stack:edit.
- Docs: drop a missing screenshot reference and correct the protected-file
  delete behavior (stack-root compose/.env cannot be deleted via the explorer).

frontend/src/components/files/MoveFileDialog.tsx

@@ -21,6 +21,8 @@ interface MoveFileDialogProps {
   relPath: string;
   /** The entry being moved (null until a source is chosen). */
   entry: FileEntry | null;
+  /** The selected file root; the destination tree is loaded within it. */
+  rootId?: string;
   /** Relocate `fromRel` into `destDir` (''=stack root). Resolves true only when
    *  the entry actually moved, so the dialog stays open on a blocked/failed move. */
   onMove: (fromRel: string, entryName: string, destDir: string) => boolean | Promise<boolean>;
@@ -32,6 +34,7 @@ export function MoveFileDialog({
   stackName,
   relPath,
   entry,
+  rootId,
   onMove,
 }: MoveFileDialogProps) {
   // Loaded directory children, keyed by directory rel path ('' = stack root).
@@ -66,7 +69,7 @@ export function MoveFileDialog({
       next.delete(dir);
       return next;
     });
-    listStackDirectory(stackName, dir)
+    listStackDirectory(stackName, dir, rootId)
       .then((entries) => {
         if (requestSeqRef.current !== seq) return;
         setDirChildren((prev) => new Map(prev).set(dir, entries.filter((e) => e.type === 'directory')));