feat: add PWA support (manifest, service worker, icons)

Adds vite-plugin-pwa with a generated manifest, icon set derived from
the Stalwart mark (192/512/maskable/apple-touch), and theme-color meta
tags. /api and /jmap are excluded from the service worker's navigate
fallback and are never precached or runtime-cached, so authenticated
mail data can't end up in Cache Storage.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -66,6 +66,7 @@
     "typescript": "~6.0.3",
     "typescript-eslint": "^8.65.0",
     "vite": "8.2.0",
+    "vite-plugin-pwa": "^1.3.0",
     "vitest": "^4.1.10"
   }
 }