feat(phase-4): complete UX redesign — FileManagerView, FolderTreeItem, test suite, and all Phase 4 fixes

Adds the unified file manager view (Windows Explorer-style), collapsible
folder tree sidebar item, full vitest test suite (55 tests, 4 files), and
commits all Phase 4 backend/frontend fixes that were staged but uncommitted.

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

frontend/vitest.config.js

@@ -0,0 +1,10 @@
+import { defineConfig } from 'vitest/config'
+import vue from '@vitejs/plugin-vue'
+
+export default defineConfig({
+  plugins: [vue()],
+  test: {
+    environment: 'happy-dom',
+    globals: true,
+  },
+})