feat(branding): install @vite-pwa/assets-generator + config

Adds pwa-assets.config.ts that reads $BRAND_DIR (default
./branding/default) and $BRAND_APP (optional per-standalone
override), resolves logo.svg/logo.png with documented fallback
order, and emits the existing icon set (favicon.ico,
icon-{192,512}.png, icon-maskable-{192,512}.png,
apple-touch-icon.png) into public/icons/.

Generator outputs alongside its source, so the config stages the
brand source into public/icons/.brand-source.{svg,png}; gitignoring
public/icons/ covers both staged source and generated icons in one
line.

Adds pnpm script `generate-pwa-assets`. Vite configs / HTML <link>
href updates come in follow-up commits; this commit alone produces
the icon set under public/icons/ but doesn't yet replace the
committed public/*.png binaries.

Part of aiolabs/webapp#95 (brand kit architecture).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -5,6 +5,7 @@
   "type": "module",
   "main": "electron/main.cjs",
   "scripts": {
+    "generate-pwa-assets": "pwa-assets-generator",
     "dev": "vite --host",
     "build": "vue-tsc -b && vite build",
     "preview": "vite preview --host",
@@ -92,6 +93,7 @@
     "@types/node": "^22.18.1",
     "@types/qrcode": "^1.5.5",
     "@types/rollup-plugin-visualizer": "^4.2.3",
+    "@vite-pwa/assets-generator": "^1.0.2",
     "@vitejs/plugin-vue": "^5.2.1",
     "@vue/tsconfig": "^0.7.0",
     "concurrently": "^8.2.2",