feat: add connection colors, query history, SQL autocomplete, and EXPLAIN visualizer

Add four developer/QA features:
- Connection color coding: color picker in dialog, colored indicators across toolbar, tabs, status bar, and connection selectors
- Query history: Rust backend with JSON file storage (500 entry cap), sidebar panel with search/clear, auto-recording from workspace
- Schema-aware SQL autocomplete: backend fetches column metadata, CodeMirror receives schema namespace with public tables unprefixed
- EXPLAIN ANALYZE visualizer: recursive tree view with cost-colored bars, expand/collapse nodes, buffers info, Results/Explain tab toggle

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

src-tauri/src/lib.rs

@@ -34,6 +34,7 @@ pub fn run() {
             commands::schema::get_table_columns,
             commands::schema::get_table_constraints,
             commands::schema::get_table_indexes,
+            commands::schema::get_completion_schema,
             // data
             commands::data::get_table_data,
             commands::data::update_row,
@@ -42,6 +43,10 @@ pub fn run() {
             // export
             commands::export::export_csv,
             commands::export::export_json,
+            // history
+            commands::history::add_history_entry,
+            commands::history::get_history,
+            commands::history::clear_history,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");