Add Phases 3.2-4.3: STT, stories, syllables, gamification

Phase 3.2 — MicrophoneInput.tsx: Browser Web Speech API for
speech-to-text recognition (EN+DE), integrated for pronunciation practice.

Phase 4.1 — Story Generator: LLM-powered mini-stories using vocabulary
words, with highlighted vocab in HTML output. Backend endpoint
POST /learning-units/{id}/generate-story + frontend /learn/[unitId]/story.

Phase 4.2 — SyllableBow.tsx: SVG arc component for syllable visualization
under words, clickable for per-syllable TTS.

Phase 4.3 — Gamification system:
- CoinAnimation.tsx: Floating coin rewards with accumulator
- CrownBadge.tsx: Crown/medal display for milestones
- ProgressRing.tsx: Circular progress indicator
- progress_api.py: Backend tracking coins, crowns, streaks per unit

Also adds "Geschichte" exercise type button to UnitCard.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend-lehrer/main.py

@@ -106,6 +106,10 @@ app.include_router(correction_router, prefix="/api")
 from learning_units_api import router as learning_units_router
 app.include_router(learning_units_router, prefix="/api")
 
+# --- 4b. Learning Progress ---
+from progress_api import router as progress_router
+app.include_router(progress_router, prefix="/api")
+
 from unit_api import router as unit_router
 app.include_router(unit_router)  # Already has /api/units prefix