chore(android): migrate app to Built-in Kotlin (drop standalone Kotlin Gradle Plugin)

## Problem
The Android app still applies the standalone **Kotlin Gradle Plugin (KGP)**, which Flutter is deprecating. Building logs:

> WARNING: Your Android app project applies the Kotlin Gradle Plugin, which will cause build failures in future versions of Flutter. Please migrate your app to **Built-in Kotlin**.

`android/app/build.gradle` still has:
- `id "kotlin-android"` (line 3)
- a `kotlinOptions { ... }` block (line 40)
- `main.java.srcDirs += 'src/main/kotlin'` (line 45)

Migration guide: https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin/for-app-developers

## Current versions (post-#598 — already upgraded)
- AGP **9.0.1**, Gradle **9.1.0**, Kotlin **2.2.20**, Flutter **3.44.6**
- So this is *not* a version bump — it's the plugin-application migration only.

## Also flagged: plugins that apply KGP
Flutter warns these dependencies apply KGP and could block future builds until each ships a Built-in-Kotlin-compatible release:

`audio_streamer, camera_android_camerax, device_info_plus, flutter_activity_recognition, flutter_background, flutter_timezone, flutter_web_auth_2, health, light, location, network_info_plus, oidc_android, open_settings_plus, package_info_plus, pedometer, polar`

We can only migrate the app project ourselves; for the plugin warnings we depend on upstream releases — track and bump as compatible versions land, and report/patch any that lag.

## Acceptance criteria
- [ ] `android/app/build.gradle` migrated to Built-in Kotlin per the guide (no standalone `kotlin-android` apply)
- [ ] App builds on Android in release mode with no KGP deprecation warning for the app project
- [ ] Audit the listed plugins; bump those with Built-in-Kotlin-compatible releases, note the blockers for the rest

## Related
- #598 — Gradle/Kotlin/AGP version upgrade (merged; this is the follow-up plugin migration)

Read the migration guide and inspect android/app/build.gradle, especially the kotlin-android, kotlinOptions, and main.java.srcDirs entries. Build the Android app in release mode, then audit the listed plugins for Built-in-Kotlin-compatible releases. Done means the app has no standalone KGP warning and remaining plugin blockers are recorded.

Written by the indexing model from the issue text.