feat: dynamic PWA manifest with configurable name, description and icons

- Add app/manifest.ts to serve /manifest.webmanifest dynamically at runtime
- Name, short_name, description, theme_color and background_color are read
  from env vars (APP_NAME, APP_SHORT_NAME, APP_DESCRIPTION, PWA_THEME_COLOR,
  PWA_BACKGROUND_COLOR) with Bulwark defaults as fallback
- Add /api/pwa-icon/[size] route that auto-generates 192x192 and 512x512 PNG
  icons from PWA_ICON_URL (or FAVICON_URL as fallback) using Sharp; results
  are cached in memory
- Remove static manifest: '/manifest.json' from layout metadata; Next.js
  injects the link automatically from app/manifest.ts
- Fix pre-existing ESLint no-undef on RequestInit in browser-navigation.ts

lib/browser-navigation.ts

@@ -57,6 +57,7 @@ export function getPathPrefix(locale?: string): string {
  *   // Browser at /webmail/en/inbox  → /webmail/api/jmap
  *   // Browser at /en/inbox          → /api/jmap
  */
+// eslint-disable-next-line no-undef
 export function apiFetch(input: string, init?: RequestInit): Promise<Response> {
   if (input.startsWith('/') && !input.startsWith('//')) {
     return fetch(getPathPrefix() + input, init);