fix: PWA install prompt - add PNG icons and fix apple-touch-icon

Browsers require PNG icons (192x192, 512x512) in the manifest to show
the install prompt. SVG-only manifests are not sufficient. Also adds the
180x180 apple-touch-icon PNG for iOS home screen, bumps SW cache to v3,
and adds scope to the manifest.

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

static/manifest.json

@@ -3,10 +3,21 @@
   "short_name": "INTERCEPT",
   "description": "Unified SIGINT platform for software-defined radio analysis",
   "start_url": "/",
+  "scope": "/",
   "display": "standalone",
   "background_color": "#0b1118",
   "theme_color": "#0b1118",
   "icons": [
+    {
+      "src": "/static/icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "/static/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png"
+    },
     {
       "src": "/static/icons/icon.svg",
       "sizes": "any",