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-tauri/src/commands/data.rs

@@ -107,6 +107,10 @@ pub async fn update_row(
     column: String,
     value: Value,
 ) -> TuskResult<()> {
+    if state.is_read_only(&connection_id).await {
+        return Err(TuskError::ReadOnly);
+    }
+
     let pools = state.pools.read().await;
     let pool = pools
         .get(&connection_id)
@@ -148,6 +152,10 @@ pub async fn insert_row(
     columns: Vec<String>,
     values: Vec<Value>,
 ) -> TuskResult<()> {
+    if state.is_read_only(&connection_id).await {
+        return Err(TuskError::ReadOnly);
+    }
+
     let pools = state.pools.read().await;
     let pool = pools
         .get(&connection_id)
@@ -184,6 +192,10 @@ pub async fn delete_rows(
     pk_columns: Vec<String>,
     pk_values_list: Vec<Vec<Value>>,
 ) -> TuskResult<u64> {
+    if state.is_read_only(&connection_id).await {
+        return Err(TuskError::ReadOnly);
+    }
+
     let pools = state.pools.read().await;
     let pool = pools
         .get(&connection_id)