fix(chat): add saved AI defaults and harden suggestions

.memory/sessions/continuity.md

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+# Session Continuity
+
+## Last Session (2026-03-09)
+- Completed `chat-provider-fixes` change set with three workstreams:
+  - `chat-loop-hardening`: invalid required-arg tool calls now terminate cleanly, not replayed, assistant tool_call history trimmed consistently
+  - `default-ai-settings`: Settings page saves default provider/model via `UserAISettings`; DB defaults authoritative over localStorage; backend fallback uses saved defaults
+  - `suggestion-add-flow`: day suggestions use resolved provider/model (not hardcoded OpenAI); modal normalizes suggestion payloads for add-to-itinerary
+- All three workstreams passed reviewer + tester validation
+- Documentation updated for all three workstreams
+
+## Active Work
+- `chat-provider-fixes` plan complete — all workstreams implemented, reviewed, tested, documented
+- See [plans/](../plans/) for other active feature plans
+- Pre-release policy established — architecture-level changes allowed (see AGENTS.md)
+
+## Known Follow-up Items (from tester findings)
+- No automated test coverage for `UserAISettings` CRUD + precedence logic
+- No automated test coverage for `send_message` streaming loop (tool error short-circuit, multi-tool partial success, `MAX_TOOL_ITERATIONS`)
+- No automated test coverage for `DaySuggestionsView.post()` 
+- `get_weather` error `"dates must be a non-empty list"` does not trigger tool-error short-circuit (mitigated by `MAX_TOOL_ITERATIONS`)
+- LLM-generated name/location fields not truncated to `max_length=200` before `LocationSerializer` (low risk)