fix(branding): wrap generator to clean up staged brand source

pwa-assets.config.ts stages the brand logo in public/icons/.brand-source.*
so the CLI (which emits next to its source) writes alongside it. Without
cleanup, the full-resolution 1024+ source ends up in dist/icons/ on every
build and is publicly served at /icons/.brand-source.png.

scripts/generate-pwa-assets.mjs runs the CLI, then removes the staged
source. Wire it through the `generate-pwa-assets` pnpm script.

Part of aiolabs/webapp#95.

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

package.json

@@ -5,7 +5,7 @@
   "type": "module",
   "main": "electron/main.cjs",
   "scripts": {
-    "generate-pwa-assets": "pwa-assets-generator",
+    "generate-pwa-assets": "node scripts/generate-pwa-assets.mjs",
     "dev": "vite --host",
     "build": "vue-tsc -b && vite build",
     "preview": "vite preview --host",