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-tauri/src/lib.rs

@@ -94,6 +94,8 @@ pub fn run() {
             commands::ai::save_ai_settings,
             commands::ai::list_ollama_models,
             commands::ai::generate_sql,
+            commands::ai::explain_sql,
+            commands::ai::fix_sql_error,
             // lookup
             commands::lookup::entity_lookup,
         ])