feat: add per-connection read-only mode

Connections default to read-only. SQL editor wraps queries in a
read-only transaction so PostgreSQL rejects mutations. Data mutation
commands (update_row, insert_row, delete_rows) are blocked at the
Rust layer. Toolbar toggle with confirmation dialog lets users
switch to read-write. Badges shown in workspace, table viewer, and
status bar.

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

src/components/table-viewer/TableDataView.tsx

@@ -7,8 +7,9 @@ import { Input } from "@/components/ui/input";
 import { updateRow as updateRowApi } from "@/lib/tauri";
 import { getTableColumns } from "@/lib/tauri";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
+import { useAppStore } from "@/stores/app-store";
 import { toast } from "sonner";
-import { Save, RotateCcw, Filter, Loader2 } from "lucide-react";
+import { Save, RotateCcw, Filter, Loader2, Lock } from "lucide-react";
 
 interface Props {
   connectionId: string;
@@ -17,6 +18,9 @@ interface Props {
 }
 
 export function TableDataView({ connectionId, schema, table }: Props) {
+  const readOnlyMap = useAppStore((s) => s.readOnlyMap);
+  const isReadOnly = readOnlyMap[connectionId] ?? true;
+
   const [page, setPage] = useState(1);
   const [pageSize, setPageSize] = useState(50);
   const [sortColumn, _setSortColumn] = useState<string | undefined>();
@@ -57,6 +61,12 @@ export function TableDataView({ connectionId, schema, table }: Props) {
 
   const handleCellDoubleClick = useCallback(
     (rowIndex: number, colIndex: number, value: unknown) => {
+      if (isReadOnly) {
+        toast.warning("Read-only mode is active", {
+          description: "Switch to Read-Write mode to edit data.",
+        });
+        return;
+      }
       const key = `${rowIndex}:${colIndex}`;
       const currentValue = pendingChanges.get(key)?.value ?? value;
       const newVal = prompt("Edit value (leave empty and click NULL for null):",
@@ -69,7 +79,7 @@ export function TableDataView({ connectionId, schema, table }: Props) {
         });
       }
     },
-    [pendingChanges]
+    [pendingChanges, isReadOnly]
   );
 
   const handleCommit = async () => {
@@ -121,6 +131,12 @@ export function TableDataView({ connectionId, schema, table }: Props) {
   return (
     <div className="flex h-full flex-col">
       <div className="flex items-center gap-2 border-b px-2 py-1">
+        {isReadOnly && (
+          <span className="flex items-center gap-1 rounded bg-yellow-500/10 px-1.5 py-0.5 text-[10px] font-semibold text-yellow-600 dark:text-yellow-500">
+            <Lock className="h-3 w-3" />
+            Read-Only
+          </span>
+        )}
         <Filter className="h-3.5 w-3.5 text-muted-foreground" />
         <Input
           placeholder="WHERE clause (e.g. id > 10)"