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/manifest.ts

@@ -0,0 +1,53 @@
+import type { MetadataRoute } from "next";
+
+export default function manifest(): MetadataRoute.Manifest {
+  const appName =
+    process.env.APP_NAME ||
+    process.env.NEXT_PUBLIC_APP_NAME ||
+    "Bulwark Webmail";
+
+  const shortName = process.env.APP_SHORT_NAME || appName;
+  const description =
+    process.env.APP_DESCRIPTION ||
+    "A modern webmail client built for Stalwart Mail Server";
+  const themeColor = process.env.PWA_THEME_COLOR || "#ffffff";
+  const backgroundColor = process.env.PWA_BACKGROUND_COLOR || "#ffffff";
+
+  // If PWA_ICON_URL or FAVICON_URL is configured, serve dynamically resized PNGs
+  // via /api/pwa-icon/[size]. Otherwise fall back to the default Bulwark PNGs.
+  const hasCustomIcon = !!(process.env.PWA_ICON_URL || process.env.FAVICON_URL);
+
+  const icons: MetadataRoute.Manifest["icons"] = hasCustomIcon
+    ? [
+        { src: "/api/pwa-icon/192", sizes: "192x192", type: "image/png", purpose: "any" },
+        { src: "/api/pwa-icon/512", sizes: "512x512", type: "image/png", purpose: "any" },
+        { src: "/api/pwa-icon/192", sizes: "192x192", type: "image/png", purpose: "maskable" },
+        { src: "/api/pwa-icon/512", sizes: "512x512", type: "image/png", purpose: "maskable" },
+      ]
+    : [
+        { src: "/icon-192x192.png", sizes: "192x192", type: "image/png", purpose: "any" },
+        { src: "/icon-512x512.png", sizes: "512x512", type: "image/png", purpose: "any" },
+        { src: "/icon-maskable-light-192x192.png", sizes: "192x192", type: "image/png", purpose: "maskable" },
+        { src: "/icon-maskable-light-512x512.png", sizes: "512x512", type: "image/png", purpose: "maskable" },
+        { src: "/icon-maskable-dark-192x192.png", sizes: "192x192", type: "image/png", purpose: "maskable" },
+        { src: "/icon-maskable-dark-512x512.png", sizes: "512x512", type: "image/png", purpose: "maskable" },
+      ];
+
+  return {
+    name: appName,
+    short_name: shortName,
+    description,
+    start_url: "/",
+    scope: "/",
+    display: "standalone",
+    orientation: "portrait-primary",
+    theme_color: themeColor,
+    background_color: backgroundColor,
+    icons,
+    categories: ["productivity"],
+    screenshots: [
+      { src: "/screenshot-540x720.png", sizes: "540x720", type: "image/png" },
+      { src: "/screenshot-1280x720.png", sizes: "1280x720", type: "image/png" },
+    ],
+  };
+}