docs: add translation model installation guide and fix plan for #6

[dskarasev]

Summary

Add comprehensive user documentation and implementation plan for fixing the 'Unable to translate' error in translation feature.

Changes

• README.md: Add Troubleshooting section with quick steps to install translation language models
• docs/LANGUAGE_MODELS.md: New detailed guide covering:
  • How translation models work on macOS
  • Step-by-step installation via System Settings
  • Verification and language selection
  • Troubleshooting for common issues
  • Advanced terminal commands
• docs/plans/2026-06-13-001-fix-unable-to-translate-bug-plan.md: Implementation plan including:
  • Root cause analysis (missing language models on device)
  • 6 implementation units for fixing error handling and improving UX
  • Consistent error handling between live overlay and region capture
  • Test scenarios and verification strategy

Why

The translation feature fails with "Unable to translate" when a language's translation model isn't installed on the Mac. This PR provides:

1. Immediate user guidance via README and docs so users can fix the issue now
2. Implementation plan for improving in-app error messages to point users to these docs
3. Root cause analysis from code investigation (per-line language detection can resolve to uninstalled models)

Related

Fixes #6

Testing

• User can read README → Troubleshooting and follow steps to install language models
• docs/LANGUAGE_MODELS.md provides detailed guidance for various scenarios
• Implementation plan is ready for code changes in follow-up PR

[PangMo5]

Thanks a lot for your first contribution!
LGTM 👍

Some context on the plan for when the actual code work happens:

• RegionCaptureFeature already has lastError, but RegionResultView renders imageData first, so a translation error doesn't really reach that branch as-is.
• TranslationError is already a type in the Translation framework, so there'd be a name clash with a new one.
• Root cause looks right. One thing to note: downloading a missing language goes through a translationTask session (prepareTranslation) rather than installedSource, so it's a bit more involved than just the error message.
• Minor: since this is docs-only, Refs #6 might fit better than Fixes #6.