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>

.gitignore

@@ -29,6 +29,7 @@ dist.tar.gz
 
 # auto-generated build file for PWA
 dev-dist/sw.js
+public/icons/
 aio-shadcn-vite.code-workspace
 dev-dist
 .specstory/history