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/models/history.rs

@@ -0,0 +1,15 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct HistoryEntry {
+    pub id: String,
+    pub connection_id: String,
+    pub connection_name: String,
+    pub database: String,
+    pub sql: String,
+    pub status: String,
+    pub error_message: Option<String>,
+    pub row_count: Option<i64>,
+    pub execution_time_ms: f64,
+    pub executed_at: String,
+}