Add custom word entry + language pair support for learning units

- New UnitBuilder component with language pair selector (DE⇄EN, ES, FR, etc.)
- Manual word entry form with auto-suggest from Kaikki dictionary (6M words)
- "No results" prompt to add multi-word terms (e.g. "schottisches Hochland")
- New backend endpoint GET /vocabulary/lookup-translation (any→any via EN hub)
- Updated POST /vocabulary/units: accepts custom_words + source_lang/target_lang
- Split unit endpoints into vocabulary/unit_api.py (500 LOC budget)

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

backend-lehrer/main.py

@@ -119,6 +119,10 @@ app.include_router(progress_router, prefix="/api")
 from vocabulary.api import router as vocabulary_router
 app.include_router(vocabulary_router, prefix="/api")
 
+# --- 4c2. Vocabulary Unit Creation + Translation ---
+from vocabulary.unit_api import router as vocab_unit_router
+app.include_router(vocab_unit_router, prefix="/api")
+
 # --- 4d. User Language Preferences ---
 from api.user_language import router as user_language_router
 app.include_router(user_language_router, prefix="/api")