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/commands/ai.rs

@@ -73,16 +73,13 @@ pub async fn list_ollama_models(ollama_url: String) -> TuskResult<Vec<OllamaMode
     Ok(tags.models)
 }
 
-#[tauri::command]
-pub async fn generate_sql(
-    app: AppHandle,
-    state: State<'_, Arc<AppState>>,
-    connection_id: String,
-    prompt: String,
+async fn call_ollama_chat(
+    app: &AppHandle,
+    system_prompt: String,
+    user_content: String,
 ) -> TuskResult<String> {
-    // Load AI settings
     let settings = {
-        let path = get_ai_settings_path(&app)?;
+        let path = get_ai_settings_path(app)?;
         if !path.exists() {
             return Err(TuskError::Ai(
                 "No AI model selected. Open AI settings to choose a model.".to_string(),
@@ -98,24 +95,6 @@ pub async fn generate_sql(
         ));
     }
 
-    // Build schema context
-    let schema_text = build_schema_context(&state, &connection_id).await?;
-
-    let system_prompt = format!(
-        "You are a PostgreSQL SQL generator. Given the database schema below and a natural language request, \
-         output ONLY a valid PostgreSQL SQL query. Do not include any explanation, markdown formatting, \
-         or code fences. Output raw SQL only.\n\n\
-         RULES:\n\
-         - Use FK relationships for correct JOIN conditions.\n\
-         - timestamp - timestamp = interval. To get a number use EXTRACT(EPOCH FROM (ts1 - ts2)).\n\
-         - interval cannot be cast to numeric directly.\n\
-         - When using UNION/UNION ALL, ensure matching column types; cast enums to text if they differ.\n\
-         - Use COALESCE for nullable columns in aggregations when appropriate.\n\
-         - Prefer LEFT JOIN when the related row may not exist.\n\n\
-         DATABASE SCHEMA:\n{}",
-        schema_text
-    );
-
     let request = OllamaChatRequest {
         model: settings.model,
         messages: vec![
@@ -125,7 +104,7 @@ pub async fn generate_sql(
             },
             OllamaChatMessage {
                 role: "user".to_string(),
-                content: prompt,
+                content: user_content,
             },
         ],
         stream: false,
@@ -162,8 +141,89 @@ pub async fn generate_sql(
         .await
         .map_err(|e| TuskError::Ai(format!("Failed to parse Ollama response: {}", e)))?;
 
-    let sql = clean_sql_response(&chat_resp.message.content);
-    Ok(sql)
+    Ok(chat_resp.message.content)
+}
+
+#[tauri::command]
+pub async fn generate_sql(
+    app: AppHandle,
+    state: State<'_, Arc<AppState>>,
+    connection_id: String,
+    prompt: String,
+) -> TuskResult<String> {
+    let schema_text = build_schema_context(&state, &connection_id).await?;
+
+    let system_prompt = format!(
+        "You are a PostgreSQL SQL generator. Given the database schema below and a natural language request, \
+         output ONLY a valid PostgreSQL SQL query. Do not include any explanation, markdown formatting, \
+         or code fences. Output raw SQL only.\n\n\
+         RULES:\n\
+         - Use FK relationships for correct JOIN conditions.\n\
+         - timestamp - timestamp = interval. To get a number use EXTRACT(EPOCH FROM (ts1 - ts2)).\n\
+         - interval cannot be cast to numeric directly.\n\
+         - When using UNION/UNION ALL, ensure matching column types; cast enums to text if they differ.\n\
+         - Use COALESCE for nullable columns in aggregations when appropriate.\n\
+         - Prefer LEFT JOIN when the related row may not exist.\n\n\
+         DATABASE SCHEMA:\n{}",
+        schema_text
+    );
+
+    let raw = call_ollama_chat(&app, system_prompt, prompt).await?;
+    Ok(clean_sql_response(&raw))
+}
+
+#[tauri::command]
+pub async fn explain_sql(
+    app: AppHandle,
+    state: State<'_, Arc<AppState>>,
+    connection_id: String,
+    sql: String,
+) -> TuskResult<String> {
+    let schema_text = build_schema_context(&state, &connection_id).await?;
+
+    let system_prompt = format!(
+        "You are a PostgreSQL expert. Explain what this SQL query does in clear, concise language. \
+         Focus on the business logic, mention the tables, joins, and filters used. \
+         Use short paragraphs or bullet points.\n\n\
+         DATABASE SCHEMA:\n{}",
+        schema_text
+    );
+
+    call_ollama_chat(&app, system_prompt, sql).await
+}
+
+#[tauri::command]
+pub async fn fix_sql_error(
+    app: AppHandle,
+    state: State<'_, Arc<AppState>>,
+    connection_id: String,
+    sql: String,
+    error_message: String,
+) -> TuskResult<String> {
+    let schema_text = build_schema_context(&state, &connection_id).await?;
+
+    let system_prompt = format!(
+        "You are a PostgreSQL expert. Fix the SQL query based on the error message. \
+         Output ONLY the corrected valid PostgreSQL SQL. Do not include any explanation, \
+         markdown formatting, or code fences. Output raw SQL only.\n\n\
+         RULES:\n\
+         - Use FK relationships for correct JOIN conditions.\n\
+         - timestamp - timestamp = interval. To get a number use EXTRACT(EPOCH FROM (ts1 - ts2)).\n\
+         - interval cannot be cast to numeric directly.\n\
+         - When using UNION/UNION ALL, ensure matching column types; cast enums to text if they differ.\n\
+         - Use COALESCE for nullable columns in aggregations when appropriate.\n\
+         - Prefer LEFT JOIN when the related row may not exist.\n\n\
+         DATABASE SCHEMA:\n{}",
+        schema_text
+    );
+
+    let user_content = format!(
+        "Original SQL:\n{}\n\nError:\n{}",
+        sql, error_message
+    );
+
+    let raw = call_ollama_chat(&app, system_prompt, user_content).await?;
+    Ok(clean_sql_response(&raw))
 }
 
 async fn build_schema_context(

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,
         ])