feat: add AI Explain Query and Fix Error via Ollama

Extract shared call_ollama_chat helper from generate_sql to reuse
settings loading and Ollama API call logic. Add two new AI commands:
- explain_sql: explains what a SQL query does in plain language
- fix_sql_error: suggests corrected SQL based on the error and schema

UI additions: "AI Explain" toolbar button, "Explain" and "Fix with AI"
action buttons on query errors, inline explanation display in results.

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

src/lib/tauri.ts

@@ -254,6 +254,12 @@ export const listOllamaModels = (ollamaUrl: string) =>
 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 });
+
 // Entity Lookup
 export const entityLookup = (
   config: ConnectionConfig,