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/components/connections/ConnectionSelector.tsx

@@ -21,18 +21,36 @@ export function ConnectionSelector() {
     );
   }
 
+  const activeConn = connectedList.find((c) => c.id === activeConnectionId);
+
   return (
     <Select
       value={activeConnectionId ?? undefined}
       onValueChange={setActiveConnectionId}
     >
       <SelectTrigger className="h-7 w-[200px] text-xs">
-        <SelectValue placeholder="Select connection" />
+        <div className="flex items-center gap-1.5">
+          {activeConn?.color && (
+            <span
+              className="h-2.5 w-2.5 shrink-0 rounded-full"
+              style={{ backgroundColor: activeConn.color }}
+            />
+          )}
+          <SelectValue placeholder="Select connection" />
+        </div>
       </SelectTrigger>
       <SelectContent>
         {connectedList.map((conn) => (
           <SelectItem key={conn.id} value={conn.id}>
-            {conn.name}
+            <div className="flex items-center gap-1.5">
+              {conn.color && (
+                <span
+                  className="h-2.5 w-2.5 shrink-0 rounded-full"
+                  style={{ backgroundColor: conn.color }}
+                />
+              )}
+              {conn.name}
+            </div>
           </SelectItem>
         ))}
       </SelectContent>