Add model dropdowns to settings and set qwen2.5:1.5b as intent default

- Default OLLAMA_INTENT_MODEL to qwen2.5:1.5b in code instead of empty
- Add GET /api/settings/models endpoint returning installed models and defaults
- Validate intent_model against installed models on save (same as default_model)
- Replace intent model text input with a dropdown of installed models
- Add chat model dropdown to Assistant settings section

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/config.py

@@ -26,7 +26,7 @@ class Config:
     OLLAMA_MODEL: str = os.environ.get("OLLAMA_MODEL", "qwen3")
     # Optional dedicated model for intent classification (should be small/fast).
     # Falls back to OLLAMA_MODEL if not set. Can also be overridden per-user via settings.
-    OLLAMA_INTENT_MODEL: str = os.environ.get("OLLAMA_INTENT_MODEL", "")
+    OLLAMA_INTENT_MODEL: str = os.environ.get("OLLAMA_INTENT_MODEL", "qwen2.5:1.5b")
     # KV cache context window for generation. Lower = less VRAM, less throughput impact.
     # 8192 is sufficient for most conversations; raise if you paste large documents.
     OLLAMA_NUM_CTX: int = int(os.environ.get("OLLAMA_NUM_CTX", "8192"))