fix: allow file tree access on Windows cross-drive project paths

FileTree component was not passing baseDir to the file API, causing
it to fall back to restricting access to the home directory. On
Windows, projects on a different drive (e.g., D:\) were blocked
with 403.

Changes:
- FileTree now passes workingDirectory as baseDir parameter
- File API routes no longer require baseDir to be under homeDir
  (allows cross-drive access on Windows)
- Still prevents using filesystem root as baseDir for safety
- Add isRootPath() utility for root path detection

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/project/FileTree.tsx

@@ -118,7 +118,7 @@ export function FileTree({ workingDirectory, onFileSelect, onFileAdd }: FileTree
     setLoading(true);
     try {
       const res = await fetch(
-        `/api/files?dir=${encodeURIComponent(workingDirectory)}&depth=4`
+        `/api/files?dir=${encodeURIComponent(workingDirectory)}&baseDir=${encodeURIComponent(workingDirectory)}&depth=4`
       );
       if (res.ok) {
         const data = await res.json();