fix(app): add Vite dev workflow, fix onLayout prop, fix icon path, allow Google Fonts CSP

The v5.0.0 React UI redesign shipped without a working dev workflow —
`npm start` only ran `electron .` which loaded the source index.html
that references raw .tsx files. The browser couldn't execute them, so
the app rendered an empty #root and the user saw 'lot of errors'.

This commit restores a real dev workflow:

1. **Vite dev server wired into npm scripts** — added `concurrently`
   + `wait-on` as devDeps. `npm run dev` now runs Vite (port 5173)
   and Electron together with `wait-on tcp:5173` so Electron only
   starts after the dev server is ready.

2. **Main process loads Vite URL in dev, dist/ in prod** — window
   module now checks `process.env.VITE_DEV_SERVER_URL` and
   `app.isPackaged` to decide what to load. Production still
   loads the built `dist/renderer/index.html`.

3. **Fixed ReferenceError: app is not defined** — window module
   referenced `app.isPackaged` but only imported `BrowserWindow`.
   Added `app` to the destructured import.

4. **Fixed wrong icon path** — window creation used
   `../../assets/icon.png` from src/main/window/ which resolved
   to `src/assets/icon.png` (doesn't exist). Icon lives at
   project root, so path is now `../../../assets/icon.png`.

5. **Allowed Google Fonts in CSP** — the strict CSP
   (`font-src 'self' data:`, `connect-src 'self'`) blocked
   the preconnect/css link to fonts.googleapis.com / .gstatic.com.
   Added both domains so Plus Jakarta Sans loads.

6. **Renamed onLayout → onLayoutChange** in AppShell — react-resizable-panels
   v4 renamed the prop. The v1 name was being spread to a DOM
   div and React logged 'Unknown event handler property' warnings.
   Test mock updated for parity.

7. **show: false → show: true** — ready-to-show was hanging on
   this Wayland/container combo. Now the window is shown
   immediately on create.

Verified end-to-end:
- 305/305 vitest tests pass
- Vite dev server starts on :5173
- Electron loads localhost:5173 and React mounts
- Header 'MarkdownConverter' + sidebar with 'No files open' visible
- No onLayout warnings after HMR
- All blocked errors resolved

package.json

@@ -5,6 +5,11 @@
   "main": "src/main/index.js",
   "scripts": {
     "start": "electron .",
+    "dev:renderer": "vite --config vite.renderer.config.ts",
+    "dev:electron": "wait-on tcp:5173 && cross-env VITE_DEV_SERVER_URL=http://localhost:5173 electron . -- --disable-gpu --disable-software-rasterizer --disable-dev-shm-usage",
+    "dev": "concurrently -k -n vite,electron -c blue,green \"npm:dev:renderer\" \"npm:dev:electron\"",
+    "build:renderer": "vite build --config vite.renderer.config.ts",
+    "preview": "npm run build:renderer && electron .",
     "test": "jest",
     "test:watch": "jest --watch",
     "test:coverage": "jest --coverage",
@@ -60,6 +65,7 @@
     "autoprefixer": "^10.5.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
+    "concurrently": "^10.0.3",
     "cross-env": "^10.0.0",
     "electron": "^41.1.1",
     "electron-builder": "^26.0.12",
@@ -78,7 +84,8 @@
     "tailwindcss-animate": "^1.0.7",
     "typescript": "^6.0.3",
     "vite": "^8.0.16",
-    "vitest": "^4.1.8"
+    "vitest": "^4.1.8",
+    "wait-on": "^9.0.10"
   },
   "dependencies": {
     "@codemirror/autocomplete": "^6.20.1",