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/lib/tauri.ts

@@ -7,6 +7,7 @@ import type {
   ColumnInfo,
   ConstraintInfo,
   IndexInfo,
+  HistoryEntry,
 } from "@/types";
 
 // Connections
@@ -125,6 +126,30 @@ export const deleteRows = (params: {
   pkValuesList: unknown[][];
 }) => invoke<number>("delete_rows", params);
 
+// History
+export const addHistoryEntry = (entry: HistoryEntry) =>
+  invoke<void>("add_history_entry", { entry });
+
+export const getHistory = (params?: {
+  connectionId?: string;
+  search?: string;
+  limit?: number;
+}) =>
+  invoke<HistoryEntry[]>("get_history", {
+    connectionId: params?.connectionId,
+    search: params?.search,
+    limit: params?.limit,
+  });
+
+export const clearHistory = () => invoke<void>("clear_history");
+
+// Completion schema
+export const getCompletionSchema = (connectionId: string) =>
+  invoke<Record<string, Record<string, string[]>>>(
+    "get_completion_schema",
+    { connectionId }
+  );
+
 // Export
 export const exportCsv = (
   path: string,