feat: retire built-in download providers, add isolated extension runtimes, Google Sans Flex font, and monochrome icon support

- Remove all built-in download provider code paths (DownloadTrack, DownloadWithFallback, tryBuiltInProvider, isBuiltInDownloadProvider, normalizeQualityForBuiltIn)
- Simplify DownloadByStrategy to route exclusively through extension providers
- Add newIsolatedExtensionRuntime() for concurrent per-download Goja VMs
- Extract reusable initializeExtensionRuntimeWithSettings() and runCleanupOnVM()
- Add TestExtensionDownloadUsesIsolatedRuntimeForConcurrentCalls
- Add Google Sans Flex font family to app themes
- Add Android adaptive icon monochrome support
- Regenerate iOS and Android app icons

android/app/src/main/res/mipmap-anydpi-v26/ic_launcher.xml

@@ -6,4 +6,9 @@
           android:drawable="@drawable/ic_launcher_foreground"
           android:inset="16%" />
   </foreground>
+  <monochrome>
+      <inset
+          android:drawable="@drawable/ic_launcher_monochrome"
+          android:inset="16%" />
+  </monochrome>
 </adaptive-icon>