chore(stack-files): client-side path-traversal guard in stackFilesApi (#1190)

The backend already rejects path-traversal attempts through
isValidRelativeStackPath, so the server side is safe today. Adding a
client-side mirror is defense-in-depth: it shortens the failure loop
(no wasted round trip) and protects against a future server-side
regression that loosens validation.

Adds isClientSafeRelPath in frontend/src/lib/stackFilesApi.ts mirroring
the backend predicate (rejects absolute paths, drive letters,
backslashes, NUL bytes, double slashes, and any segment that is `.`
or `..`). Wraps every export that accepts a relPath / targetDir /
fromRel / toRel argument with assertSafeRelPath, throwing a clear
Error before the fetch is issued.

12 unit tests cover the predicate (POSIX accepts, traversal rejects,
Windows drive letters, backslashes, NUL bytes, non-string inputs).
Frontend suite stays at 288/288.

frontend/src/lib/stackFilesApi.ts

@@ -1,5 +1,32 @@
 import { apiFetch } from './api';
 
+/**
+ * Mirrors backend/src/utils/validation.ts::isValidRelativeStackPath. Client
+ * defense-in-depth: the backend rejects path-traversal attempts, but catching
+ * them client-side avoids a wasted round trip and surfaces a clearer error
+ * to the user. Also guards against a future regression on the server side.
+ *
+ * Allow: the empty string (means the stack root) and POSIX-style relative
+ * paths with no traversal segments. Reject: absolute paths, drive letters,
+ * backslashes, double slashes, NUL bytes, and any segment that is `.` or `..`.
+ */
+export function isClientSafeRelPath(rel: string): boolean {
+  if (typeof rel !== 'string') return false;
+  if (rel === '') return true;
+  if (rel.includes('\0')) return false;
+  if (rel.includes('\\')) return false;
+  if (/^[a-zA-Z]:/.test(rel) || rel.startsWith('/')) return false;
+  if (rel.includes('//')) return false;
+  const segments = rel.split('/');
+  return !segments.some(seg => seg === '..' || seg === '.');
+}
+
+function assertSafeRelPath(rel: string, label = 'path'): void {
+  if (!isClientSafeRelPath(rel)) {
+    throw new Error(`Invalid ${label}: must be a relative path inside the stack directory`);
+  }
+}
+
 export interface FileEntry {
   name: string;
   type: 'file' | 'directory' | 'symlink';
@@ -33,6 +60,7 @@ export async function listStackDirectory(
   stackName: string,
   relPath: string
 ): Promise<FileEntry[]> {
+  assertSafeRelPath(relPath);
   const res = await apiFetch(stackFilesUrl(stackName, `?path=${encodeURIComponent(relPath)}`));
   if (!res.ok) throw new Error(await parseApiError(res));
   return res.json() as Promise<FileEntry[]>;
@@ -42,6 +70,7 @@ export async function readStackFile(
   stackName: string,
   relPath: string
 ): Promise<FileContentResult> {
+  assertSafeRelPath(relPath);
   const res = await apiFetch(stackFilesUrl(stackName, `/content?path=${encodeURIComponent(relPath)}`));
   if (!res.ok) throw new Error(await parseApiError(res));
   return res.json() as Promise<FileContentResult>;
@@ -51,6 +80,7 @@ export async function downloadStackFile(
   stackName: string,
   relPath: string
 ): Promise<Response> {
+  assertSafeRelPath(relPath);
   return apiFetch(stackFilesUrl(stackName, `/download?path=${encodeURIComponent(relPath)}`));
 }
 
@@ -60,6 +90,7 @@ export async function uploadStackFile(
   file: File,
   options?: { localOnly?: boolean }
 ): Promise<void> {
+  assertSafeRelPath(targetDir, 'target directory');
   const fd = new FormData();
   fd.append('file', file, file.name);
 
@@ -103,6 +134,7 @@ export async function writeStackFile(
   relPath: string,
   content: string
 ): Promise<void> {
+  assertSafeRelPath(relPath);
   const res = await apiFetch(
     stackFilesUrl(stackName, `/content?path=${encodeURIComponent(relPath)}`),
     { method: 'PUT', body: JSON.stringify({ content }) }
@@ -115,6 +147,7 @@ export async function deleteStackPath(
   relPath: string,
   recursive?: boolean
 ): Promise<void> {
+  assertSafeRelPath(relPath);
   const qs = recursive
     ? `path=${encodeURIComponent(relPath)}&recursive=1`
     : `path=${encodeURIComponent(relPath)}`;
@@ -126,6 +159,7 @@ export async function mkdirStackPath(
   stackName: string,
   relPath: string
 ): Promise<void> {
+  assertSafeRelPath(relPath);
   const res = await apiFetch(
     stackFilesUrl(stackName, `/folder?path=${encodeURIComponent(relPath)}`),
     { method: 'POST', body: JSON.stringify({}) }
@@ -138,6 +172,8 @@ export async function renameStackPath(
   fromRel: string,
   toRel: string
 ): Promise<void> {
+  assertSafeRelPath(fromRel, 'source path');
+  assertSafeRelPath(toRel, 'destination path');
   const res = await apiFetch(
     stackFilesUrl(stackName, '/rename'),
     { method: 'PATCH', body: JSON.stringify({ from: fromRel, to: toRel }) }
@@ -154,6 +190,7 @@ export async function getStackEntryPermissions(
   stackName: string,
   relPath: string
 ): Promise<EntryPermissions> {
+  assertSafeRelPath(relPath);
   const res = await apiFetch(stackFilesUrl(stackName, `/permissions?path=${encodeURIComponent(relPath)}`));
   if (!res.ok) throw new Error(await parseApiError(res));
   return res.json() as Promise<EntryPermissions>;
@@ -164,6 +201,7 @@ export async function setStackEntryPermissions(
   relPath: string,
   mode: number
 ): Promise<void> {
+  assertSafeRelPath(relPath);
   const res = await apiFetch(
     stackFilesUrl(stackName, `/permissions?path=${encodeURIComponent(relPath)}`),
     { method: 'PUT', body: JSON.stringify({ mode }) }