fix: Multi-language awareness bugfixes

[Peleke]

Summary

This PR fixes multiple language awareness issues in the application:

• ElevenLabs API Key (Bug: ElevenLabs API key env var mismatch causes TTS failure in production #129): Corrected env var name from ELEVEN_LABS_API_KEY to ELEVENLABS_API_KEY to match deployment config
• Exercise Direction Detection (Bug: Exercise direction detection broken for Icelandic (hardcoded language matching) #130): Made translation direction detection language-agnostic (was hardcoded for Spanish/Latin only)
• Training Ground Feedback (Bug: Training Ground exercise feedback hardcoded to Spanish #131): Added language-specific success/failure messages for exercises

Closes

Closes #129
Closes #130
Closes #131

Changes

File	Change
app/api/tts/synthesize/route.ts	Fix env var name
app/api/lessons/[lessonId]/generate-from-reading/route.ts	Language-agnostic direction detection
lib/languages/index.ts	Add getExerciseFeedback() utility
components/exercises/*.tsx	Accept language prop, use localized feedback
components/hub/ExercisesRegion.tsx	Pass language to exercise components

Test Plan

Automated Tests

• TTS route tests pass (20 tests)
• Direction detection tests pass (11 tests)
• Language config tests pass (38 tests)

Manual UAT Steps

Bug #129: ElevenLabs TTS

1. Navigate to any lesson with audio content
2. Click the audio playback button
3. Expected: Audio should play without "API key not configured" error

Bug #130: Exercise Direction Detection

1. Create a new Icelandic lesson with a reading
2. Generate exercises (include translation type)
3. Check the generated translation exercises in the database
4. Expected: direction field should be is_to_en or en_to_is, NOT containing 'spanish' or 'latin'

Bug #131: Training Ground Feedback

1. Navigate to Training Ground for a Spanish lesson

2. Complete an exercise correctly

3. Expected: See "¡Correcto!" feedback

4. Navigate to Training Ground for a Latin lesson

5. Complete an exercise correctly

6. Expected: See "Recte!" feedback

7. Navigate to Training Ground for an Icelandic lesson

8. Complete an exercise correctly

9. Expected: See "Rétt!" feedback

🤖 Generated with Claude Code

[Peleke]

Ready for Review

All automated tests pass. UAT steps are documented in the PR description.

Quick Verification Checklist

Area	Test	Status
TTS API	Unit tests	✅ Pass (20/20)
Direction Detection	Unit tests	✅ Pass (11/11)
Language Feedback	Unit tests	✅ Pass (38/38 including 3 new)
Type Check	No new errors	✅

UAT Summary

1. TTS Audio - Play audio on any lesson page (requires deployed staging to verify)
2. Icelandic Exercises - Generate exercises for Icelandic content and verify direction metadata
3. Training Ground Feedback - Test exercise completion feedback in Spanish/Latin/Icelandic lessons

[Peleke]

✅ UAT Complete

Tests Performed

Bug #129: ElevenLabs TTS

• Navigated to Latin lesson with audio content
• Clicked audio playback button
• Result: ✅ Audio plays correctly (after fixing env var ELEVEN_LABS_API_KEY → ELEVENLABS_API_KEY)

Bug #130: Exercise Direction Detection

• Created Icelandic lesson with reading content
• Generated exercises including translation type
• Verified direction field in database
• Result: ✅ Direction shows is_to_en / en_to_is (not hardcoded Spanish/Latin)

Bug #131: Training Ground Feedback

• Tested exercises for Spanish lesson → "¡Correcto!" feedback ✅
• Tested exercises for Latin lesson → "Recte!" feedback ✅
• Tested exercises for Icelandic lesson → "Rétt!" feedback ✅

Additional Testing

• Dark mode support on Profile and Flashcards pages ✅
• Mobile scroll fix verified ✅

Environment

• Tested on uat/test-all-prs branch with all 3 PRs merged
• Local dev server + Supabase

Ready to merge 🚀