refactor(ai): consolidate AI around chat tool-calling; add OpenRouter

- rework chat backend (chat.rs, chat_tools.rs, ai.rs, models, state) around tool calls
- add OpenRouter provider alongside Ollama/Fireworks in settings
- drop inline AiBar, ResultsPanel explain/fix UI and ChartPreview in favour of the chat panel
- add frontend chat tool-registry

src/lib/tauri.ts

@@ -214,14 +214,8 @@ export const listOllamaModels = (ollamaUrl: string) =>
 export const listFireworksModels = (apiKey: string) =>
   invoke<OllamaModel[]>("list_fireworks_models", { apiKey });
 
-export const generateSql = (connectionId: string, prompt: string) =>
-  invoke<string>("generate_sql", { connectionId, prompt });
-
-export const explainSql = (connectionId: string, sql: string) =>
-  invoke<string>("explain_sql", { connectionId, sql });
-
-export const fixSqlError = (connectionId: string, sql: string, errorMessage: string) =>
-  invoke<string>("fix_sql_error", { connectionId, sql, errorMessage });
+export const listOpenRouterModels = (apiKey: string) =>
+  invoke<OllamaModel[]>("list_openrouter_models", { apiKey });
 
 export const chatSend = (connectionId: string, messages: ChatMessage[]) =>
   invoke<ChatTurnResult>("chat_send", { connectionId, messages });