feat: add navigator and gallery file APIs

[Airsaid]

Summary

• Add second-batch Navigator APIs for app permission settings, speech recognition settings, text-to-speech settings, and opening ContentSource content.
• Add FileToolkit.saveImageToGallery(ContentSource) with SaveFileResult, SavedFileRef, and SaveFileFailureReason.
• Update Android/iOS implementations, tests, and English/Chinese README examples.

Key Changes

• Android Navigator now prefers Xiaomi-family permission settings before app settings, falls back speech/TTS settings to app settings, and opens bytes/files/URIs/URLs through system handlers.
• iOS Navigator falls speech/TTS/permission settings back to app settings and supports file/url/bytes content opening.
• Android Files saves images through MediaStore or legacy Pictures; iOS saves images to Photos and returns success with no stable ref when no uri/path is available.

Validation

• ./gradlew :toolkit:testAndroid
• ./gradlew :toolkit:testAndroidHostTest
• ./gradlew :toolkit:lint --no-parallel
• ./gradlew :composeApp:assembleDebug
• ./gradlew :toolkit:compileKotlinIosSimulatorArm64
• ./gradlew :toolkit:allTests
• ./gradlew :toolkit:compileKotlinIosArm64
• git diff --check