feat(tauri): add Tauri v2 Android app wrapper

- Wrap apps/web as Android APK via Tauri v2 WebView (zero web changes)
- Disable default compression feature to avoid brotli/alloc-no-stdlib conflict with Rust 1.96
- Add RGBA icon placeholders
- Add Android keystore to gitignore

APK output: apps/tauri/gen/android/app/build/outputs/apk/

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

apps/tauri/capabilities/default.json

@@ -0,0 +1,8 @@
+{
+  "identifier": "default",
+  "description": "Capability for the main window",
+  "windows": ["main"],
+  "permissions": [
+    "core:default"
+  ]
+}