feat(android): file upload/download + in-app tab redesign

Companion WebView now supports file inputs and downloads, and apps
opened in the in-app tab get a proper loading splash and a footer
control bar matching the web app-session bar.

- onShowFileChooser wired to an ActivityResultLauncher so <input
  type=file> opens the system file browser (kiosk + in-app tab)
- DownloadListener: http(s) via DownloadManager (forwarding session
  cookies), blob: via JS->base64->MediaStore, data: decoded inline
- in-app tab: app-icon + progress loading splash (eager favicon
  fetch, upgraded via onReceivedIcon)
- footer controls (back/forward/refresh/open/close) matched to the
  web AppSession mobile bar, with the same SVG glyphs as drawables
- bump to 0.4.8 (versionCode 12)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Android/app/src/main/res/drawable/ic_nav_newtab.xml

@@ -0,0 +1,12 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="24"
+    android:viewportHeight="24">
+    <path
+        android:pathData="M10,6H6a2,2 0,0 0,-2 2v10a2,2 0,0 0,2 2h10a2,2 0,0 0,2 -2v-4M14,4h6m0,0v6m0,-6L10,14"
+        android:strokeColor="#FFFFFF"
+        android:strokeWidth="2"
+        android:strokeLineCap="round"
+        android:strokeLineJoin="round" />
+</vector>