refactor(branding): switch to /icons/ paths and remove committed binaries

PWA icons now ship from public/icons/ (generated by
@vite-pwa/assets-generator, gitignored). The seven hand-crafted
binaries at public/ root come out of the tree.

Changes:
- 7 deleted: public/{favicon.ico, apple-touch-icon.png, mask-icon.svg,
  icon-{192,512}.png, icon-maskable-{192,512}.png}
- 9 HTML: <link rel="icon"|"apple-touch-icon"> hrefs prefixed with
  /icons/. mask-icon link dropped (PNG source → no sharp SVG; modern
  browsers prefer favicon.svg anyway, which we can revisit when an
  SVG brand source lands).
- 8 vite configs: includeAssets[] + manifest.icons[].src prefixed
  with icons/. Vite rewrites /icons/foo → <base>/icons/foo when base
  is set (so /events/icons/favicon.ico under /events/ deploys).

Build is now dependent on `pnpm generate-pwa-assets` running first
(or whatever invokes the generator — Phase 2 NixOS builds wire this
into buildNpmPackage). Standalone dev runs the generator on first
boot or whenever BRAND_DIR changes.

Part of aiolabs/webapp#95.

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

public/mask-icon.svg

@@ -1,3 +0,0 @@
-<svg width="16" height="16" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
-  <path d="M13 3L4 14h7l-2 7 9-11h-7l2-7z" stroke="black" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
-</svg>