[codex] Refresh critical dependencies

[jjoonleo]

Summary

• Refreshes the issue [Production readiness] Refresh stale and discontinued dependencies #394 critical dependency batch, including Firebase, Google Sign-In, GoRouter, local notifications, secure storage, AppAuth, Drift, Freezed, GetIt, and generator-related updates.
• Removes the direct discontinued js dependency and migrates the web interop declarations to dart:js_interop.
• Updates Google Sign-In v7 and flutter_local_notifications v20 call sites, plus Android/iOS platform requirements.
• Documents intentionally deferred packages in docs/Dependency-Refresh-394.md.

Platform Notes

• Android now builds with Java 17, AGP 8.6.0, Kotlin 2.0.20, compile SDK 36, and min SDK 23.
• iOS deployment target is raised to 15.0 because Firebase 12.12.0 declares iOS 15.0 as its minimum.

Validation

• flutter pub get
• dart run build_runner build --delete-conflicting-outputs
• flutter analyze
• flutter test
• flutter build web --release
• flutter build apk --debug
• pod install
• flutter build ios --debug --no-codesign

Closes #394

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