Wires the agent chat frontend — AgentChatTile with ephemeral conversation state, ChatInput with Enter-to-send, Agents settings category (endpoint/model/API key/test connection), and schema v95 migration.
Adds the Rust inference module — InferenceBackend trait, RemoteClient (OpenAI-compat HTTP), and chat/test_inference_connection Tauri commands. No frontend wiring yet; that's Commit 2.