feat: Add PWA support with manifest and ServiceWorker integration

- Mounted static files directory to serve PWA assets like manifest.json and ServiceWorker scripts.
- Updated `auth.html` and `keys_status.html` templates:
  - Added `<link>` for manifest and icons to support Progressive Web App (PWA) features.
  - Added meta tags for theme color and Apple web app capabilities.
  - Integrated ServiceWorker registration script for offline capabilities.

app/static/service-worker.js

@@ -0,0 +1,43 @@
+const CACHE_NAME = 'gbalance-cache-v1';
+const urlsToCache = [
+  '/',
+  '/static/manifest.json',
+  '/static/icons/icon-192x192.png'
+];
+
+self.addEventListener('install', event => {
+  event.waitUntil(
+    caches.open(CACHE_NAME)
+      .then(cache => {
+        console.log('Opened cache');
+        return cache.addAll(urlsToCache);
+      })
+  );
+});
+
+self.addEventListener('fetch', event => {
+  event.respondWith(
+    caches.match(event.request)
+      .then(response => {
+        if (response) {
+          return response;
+        }
+        return fetch(event.request);
+      })
+  );
+});
+
+self.addEventListener('activate', event => {
+  const cacheWhitelist = [CACHE_NAME];
+  event.waitUntil(
+    caches.keys().then(cacheNames => {
+      return Promise.all(
+        cacheNames.map(cacheName => {
+          if (cacheWhitelist.indexOf(cacheName) === -1) {
+            return caches.delete(cacheName);
+          }
+        })
+      );
+    })
+  );
+});