fix(chat): improve OpenCode Zen integration and error handling

- Fetch models dynamically from OpenCode Zen API (36 models vs 5 hardcoded)
- Add function calling support check before using tools
- Add retry logic (num_retries=2) for transient failures
- Improve logging for debugging API calls and errors
- Update system prompt for multi-stop itinerary context
- Clean up unused imports in frontend components
- Remove deleted views.py (moved to views/__init__.py)

docs/docs/usage/usage.md

@@ -26,6 +26,8 @@ The term "Location" is now used instead of "Adventure" - the usage remains the s
 
 The AI travel chat is embedded in the **Collections → Recommendations** view. Select a collection, switch to the Recommendations tab, and use the chat to brainstorm destinations, ask for travel advice, or get location suggestions. The chat supports multiple LLM providers — configure your API key in **Settings → API Keys** and pick a provider from the dropdown in the chat interface. The provider list is loaded dynamically from the backend, so any provider supported by LiteLLM (plus OpenCode Zen) is available.
 
+You can also override the default model for any provider by typing a model name in the **Model** input next to the provider selector (e.g. `openai/gpt-5-nano`). Your model choice is saved per-provider in the browser. If the model field is left empty, the provider's default model is used. Provider errors (authentication, model not found, rate limits) are displayed as clear, actionable messages in the chat.
+
 #### Collections
 
 - **Collection**: a collection is a way to group locations together. Collections are flexible and can be used in many ways. When no start or end date is added to a collection, it acts like a folder to group locations together. When a start and end date is added to a collection, it acts like a trip to group locations together that were visited during that time period. With start and end dates, the collection is transformed into a full itinerary with a timeline-style day view — each day displays numbered stops as compact cards (without image banners), connector rows between consecutive locations showing distance and travel time via OSRM routing (walking if ≤ 20 min, driving otherwise) with automatic haversine fallback when OSRM is unavailable, and a single `+ Add` control for inserting new places. Lodging placement follows directional rules: on check-in day it appears after the last stop, on check-out day it appears before the first stop, and on days with no locations a single lodging card is shown (or two cards when a checkout and checkin are different lodgings). Connector rows link lodging to adjacent locations. Day-level quick actions include Auto-fill (populates an empty itinerary from dated records) and Optimize (nearest-neighbor route ordering for coordinate-backed stops). The day date pill displays a weather temperature summary when available, with graceful fallback if weather data is unavailable. The itinerary also includes a map showing the route taken between locations. Your most recently updated collections also appear on the dashboard. For example, you could have a collection for a trip to Europe with dates so you can plan where you want to visit, a collection of local hiking trails, or a collection for a list of restaurants you want to try.