Add PWA support and Google Takeout subscription import

- manifest.json + icon.svg for installability on mobile (standalone mode)
- index.html: theme-color, apple-mobile-web-app meta tags, manifest link
- Settings: Import CSV section reads Google Takeout subscriptions.csv,
  extracts UC... channel IDs, calls follow-bulk to follow them all at once

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

frontend/public/manifest.json

@@ -0,0 +1,27 @@
+{
+  "name": "YT Hub",
+  "short_name": "YT Hub",
+  "description": "Your personal YouTube archive",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#09090b",
+  "theme_color": "#09090b",
+  "icons": [
+    {
+      "src": "/icon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png"
+    }
+  ]
+}