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

app/layout.tsx

@@ -22,7 +22,6 @@ export async function generateMetadata(): Promise<Metadata> {
   return {
     title: process.env.APP_NAME || process.env.NEXT_PUBLIC_APP_NAME || "Webmail",
     description: "Minimalist webmail client using JMAP protocol",
-    manifest: "/manifest.json",
     appleWebApp: {
       capable: true,
       statusBarStyle: "black-translucent",