PWA installability + working service worker (wishlist #8)

manifest.webmanifest with the existing icons, apple-touch-icon, and SW
registration at startup. The old sw.js was a self-unregistering cleanup
stub — meaning serviceWorker.ready never resolved and web push has been
silently dead; the new worker handles push display and notification
clicks, and makes the app installable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/public/manifest.webmanifest

@@ -0,0 +1,29 @@
+{
+  "name": "WealthySmart",
+  "short_name": "WealthySmart",
+  "description": "Smart personal finance management",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#0f172a",
+  "theme_color": "#0f172a",
+  "lang": "es-CR",
+  "icons": [
+    {
+      "src": "/icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png"
+    },
+    {
+      "src": "/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ]
+}