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/types/index.ts

@@ -53,6 +53,50 @@ export interface IndexInfo {
   is_primary: boolean;
 }
 
+export interface HistoryEntry {
+  id: string;
+  connection_id: string;
+  connection_name: string;
+  database: string;
+  sql: string;
+  status: string;
+  error_message?: string;
+  row_count?: number;
+  execution_time_ms: number;
+  executed_at: string;
+}
+
+export interface ExplainNode {
+  "Node Type": string;
+  "Relation Name"?: string;
+  "Schema"?: string;
+  "Alias"?: string;
+  "Startup Cost": number;
+  "Total Cost": number;
+  "Plan Rows": number;
+  "Plan Width": number;
+  "Actual Startup Time"?: number;
+  "Actual Total Time"?: number;
+  "Actual Rows"?: number;
+  "Actual Loops"?: number;
+  "Shared Hit Blocks"?: number;
+  "Shared Read Blocks"?: number;
+  "Filter"?: string;
+  "Join Type"?: string;
+  "Index Name"?: string;
+  "Index Cond"?: string;
+  "Hash Cond"?: string;
+  "Sort Key"?: string[];
+  Plans?: ExplainNode[];
+  [key: string]: unknown;
+}
+
+export interface ExplainResult {
+  Plan: ExplainNode;
+  "Planning Time": number;
+  "Execution Time": number;
+}
+
 export type TabType = "query" | "table" | "structure";
 
 export interface Tab {