fix: address code review - fallback to homedir, validate baseDir, use afterAll

Address all 3 points from @op7418's review:

1. No baseDir fallback (severe): Both /api/files and /api/files/preview
   now fall back to os.homedir() when no baseDir is provided, preventing
   access to arbitrary system directories/files.

2. baseDir bypass via baseDir=/ (medium): Both APIs now validate that
   baseDir itself is within the user's home directory. Setting baseDir=/
   will be rejected with 403.

3. Test cleanup (minor): Replaced 'it(cleanup...)' with proper after()
   hook. Added 6 new tests for baseDir validation (18 total, all pass).

src/app/api/files/preview/route.ts

@@ -15,14 +15,25 @@ export async function GET(request: NextRequest) {
   }
 
   const path = require('path');
+  const os = require('os');
   const resolvedPath = path.resolve(filePath);
+  const homeDir = os.homedir();
 
   // Validate that the file is within the session's working directory.
   // The baseDir parameter should be the session's working directory,
   // which acts as the trust boundary for file access.
+  // The baseDir itself must be under the user's home directory to prevent
+  // attackers from setting baseDir=/ to bypass all restrictions.
   const baseDir = searchParams.get('baseDir');
   if (baseDir) {
     const resolvedBase = path.resolve(baseDir);
+    // Ensure baseDir is within the home directory (prevent baseDir=/ bypass)
+    if (!isPathSafe(homeDir, resolvedBase)) {
+      return NextResponse.json<ErrorResponse>(
+        { error: 'Base directory is outside the allowed scope' },
+        { status: 403 }
+      );
+    }
     if (!isPathSafe(resolvedBase, resolvedPath)) {
       return NextResponse.json<ErrorResponse>(
         { error: 'File is outside the project scope' },
@@ -32,8 +43,6 @@ export async function GET(request: NextRequest) {
   } else {
     // Fallback: without a baseDir, restrict to the user's home directory
     // to prevent reading arbitrary system files like /etc/passwd
-    const os = require('os');
-    const homeDir = os.homedir();
     if (!isPathSafe(homeDir, resolvedPath)) {
       return NextResponse.json<ErrorResponse>(
         { error: 'File is outside the allowed scope' },

src/app/api/files/route.ts

@@ -15,21 +15,40 @@ export async function GET(request: NextRequest) {
   }
 
   const path = require('path');
+  const os = require('os');
   const resolvedDir = path.resolve(dir);
+  const homeDir = os.homedir();
 
   // Use baseDir (the session's working directory) as the trust boundary.
-  // If no baseDir is provided, fall back to the requested directory itself
-  // (preserves backward compatibility while still preventing traversal
-  // when the frontend supplies the session working directory).
+  // The baseDir itself must be under the user's home directory to prevent
+  // attackers from setting baseDir=/ to bypass all restrictions.
+  // If no baseDir is provided, fall back to the user's home directory
+  // to prevent scanning arbitrary system directories.
   const baseDir = searchParams.get('baseDir');
   if (baseDir) {
     const resolvedBase = path.resolve(baseDir);
+    // Ensure baseDir is within the home directory (prevent baseDir=/ bypass)
+    if (!isPathSafe(homeDir, resolvedBase)) {
+      return NextResponse.json<ErrorResponse>(
+        { error: 'Base directory is outside the allowed scope' },
+        { status: 403 }
+      );
+    }
     if (!isPathSafe(resolvedBase, resolvedDir)) {
       return NextResponse.json<ErrorResponse>(
         { error: 'Directory is outside the project scope' },
         { status: 403 }
       );
     }
+  } else {
+    // Fallback: without a baseDir, restrict to the user's home directory
+    // to prevent scanning arbitrary system directories like /etc
+    if (!isPathSafe(homeDir, resolvedDir)) {
+      return NextResponse.json<ErrorResponse>(
+        { error: 'Directory is outside the allowed scope' },
+        { status: 403 }
+      );
+    }
   }
 
   try {