feat(android): upgrade in-process WebView on devices stuck on old system WebView (#4142)

Add tauri-plugin-webview-upgrade as a git submodule under
apps/readest-app/src-tauri/plugins/. On Android devices whose system
WebView is locked to an old Chromium build (Huawei phones, Moaan / Onyx
/ Kobo e-ink readers, AOSP forks without Play Store, etc.), the reader
bundle renders as a blank screen. The plugin bootstraps before
Application.onCreate via androidx.startup and redirects the in-process
WebView loader to a recent com.google.android.webview when the user has
one sideloaded — opening the only window in which WebViewUpgrade can
swap the provider, before Tauri/Wry creates any WebView.

Thresholds (minUpgradeMajor / minSupportedMajor) come from
plugins.webview-upgrade in tauri.conf.json and are baked into Kotlin
constants at Gradle build time. Below the supported threshold with no
upgrade option, the plugin shows a localized AlertDialog (15 languages,
English fallback) prompting the user to install Android System WebView.

Plugin source: https://github.com/readest/tauri-plugin-webview-upgrade

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/readest-app/src-tauri/Cargo.toml

@@ -56,6 +56,7 @@ tauri-plugin-haptics = "2"
 tauri-plugin-persisted-scope = "2"
 tauri-plugin-native-bridge = { path = "./plugins/tauri-plugin-native-bridge" }
 tauri-plugin-native-tts = { path = "./plugins/tauri-plugin-native-tts" }
+tauri-plugin-webview-upgrade = { path = "./plugins/tauri-plugin-webview-upgrade" }
 tauri-plugin-websocket = "2"
 tauri-plugin-sharekit = "0.3"
 tauri-plugin-device-info = "1.0.1"