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/error.rs

@@ -17,6 +17,9 @@ pub enum TuskError {
     #[error("Not connected: {0}")]
     NotConnected(String),
 
+    #[error("Connection is in read-only mode")]
+    ReadOnly,
+
     #[error("{0}")]
     Custom(String),
 }