Fix sidebar content overflow and set correct window taskbar icon

The sidebar config panel content was overflowing its container width,
causing project names and directory paths to be clipped. Added min-w-0
and overflow-hidden to flex containers, and restructured folder path
config rows to stack vertically instead of cramming into one line.

The Windows taskbar was showing a black square because no default window
icon was set at runtime. Added default_window_icon() call in the Tauri
builder using the app's icon.png.

Also adds vitest test infrastructure with tests verifying both fixes.

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

app/package.json

@@ -7,7 +7,9 @@
     "dev": "vite",
     "build": "tsc && vite build",
     "preview": "vite preview",
-    "tauri": "tauri"
+    "tauri": "tauri",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "@tauri-apps/api": "^2",
@@ -25,13 +27,17 @@
   "devDependencies": {
     "@tailwindcss/vite": "^4",
     "@tauri-apps/cli": "^2",
+    "@testing-library/jest-dom": "^6.9.1",
+    "@testing-library/react": "^16.3.2",
     "@types/react": "^19.0.0",
     "@types/react-dom": "^19.0.0",
     "@vitejs/plugin-react": "^4",
     "autoprefixer": "^10",
+    "jsdom": "^28.1.0",
     "postcss": "^8",
     "tailwindcss": "^4",
     "typescript": "^5.7",
-    "vite": "^6"
+    "vite": "^6",
+    "vitest": "^4.0.18"
   }
 }