feat: add first-run setup guide

[zefutoff]

Summary

• add a first-run setup guide
• explain the three available correction modes:
  • native Android spell checker
  • PROCESS_TEXT selection correction
  • Grammalecte IME fallback
• show the real spell-checker and IME activation state
• provide direct access to the relevant Android settings
• show the guide automatically only once
• keep the guide permanently accessible from the main screen
• keep the setup guide activity internal and non-exported
• complete two Phase 4 roadmap items:
  • Improve first-run setup
  • Explain the three correction modes in-app

Behavior

On first launch:

• the setup guide opens automatically
• the user can configure the native spell checker and IME directly
• PROCESS_TEXT is explained as requiring no activation
• leaving the guide returns to the main application

On later launches:

• the guide does not reopen automatically
• it remains manually accessible from the main screen

Validation

• manually validated on Samsung Galaxy S21
• automatic first-run opening validated
• no automatic reopening on later launches
• manual reopening validated
• live spell-checker and IME states validated
• SetupGuideInstrumentedTest passes
• SetupGuideManifestTest passes
• make check
• make assemble
• git diff --check
• final APK does not request android.permission.INTERNET