fix: serve PWA manifest dynamically to respect runtime APP_NAME env var

Delete static public/manifest.json (hardcoded "Bulwark Webmail") and mark
app/manifest.ts as force-dynamic so Next.js evaluates APP_NAME at request
time instead of build time, fixing the native browser install prompt.

Closes #207

app/manifest.ts

@@ -1,5 +1,7 @@
 import type { MetadataRoute } from "next";
 
+export const dynamic = "force-dynamic";
+
 export default function manifest(): MetadataRoute.Manifest {
   const appName =
     process.env.APP_NAME ||