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/learning_units_api.py

@@ -348,3 +348,29 @@ def api_get_next_review(unit_id: str, limit: int = 5):
     except Exception as e:
         raise HTTPException(status_code=500, detail=str(e))
 
+
+class StoryGeneratePayload(BaseModel):
+    vocabulary: List[Dict[str, Any]]
+    language: str = "en"
+    grade_level: str = "5-8"
+
+
+@router.post("/{unit_id}/generate-story")
+def api_generate_story(unit_id: str, payload: StoryGeneratePayload):
+    """Generate a short story using vocabulary words."""
+    lu = get_learning_unit(unit_id)
+    if not lu:
+        raise HTTPException(status_code=404, detail="Lerneinheit nicht gefunden.")
+
+    try:
+        from story_generator import generate_story
+        result = generate_story(
+            vocabulary=payload.vocabulary,
+            language=payload.language,
+            grade_level=payload.grade_level,
+        )
+        return result
+    except Exception as e:
+        logger.error(f"Story generation failed for {unit_id}: {e}")
+        raise HTTPException(status_code=500, detail=f"Story-Generierung fehlgeschlagen: {e}")
+