fix: handle Docker build placeholder env vars in Supabase clients and VAPID init

Docker builds use __NEXT_PUBLIC_*__ sentinel values that get replaced at
runtime by docker-entrypoint.sh. These placeholders caused build failures:
- Supabase client constructor rejected invalid URL during page prerendering
- web-push VAPID init rejected invalid key format

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/supabase/client.ts

@@ -1,8 +1,16 @@
 import { createBrowserClient } from '@supabase/ssr'
 
+// During Docker builds, NEXT_PUBLIC_* vars are placeholder sentinels
+// (e.g. __NEXT_PUBLIC_SUPABASE_URL__) that get replaced at runtime by
+// docker-entrypoint.sh. Provide a dummy URL so the client constructor
+// doesn't throw during Next.js static page generation.
+const url = process.env.NEXT_PUBLIC_SUPABASE_URL!
+const key = process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY!
+const isBuildPlaceholder = url.startsWith('__')
+
 export function createClient() {
   return createBrowserClient(
-    process.env.NEXT_PUBLIC_SUPABASE_URL!,
-    process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY!
+    isBuildPlaceholder ? 'https://placeholder.supabase.co' : url,
+    isBuildPlaceholder ? 'placeholder' : key
   )
 }