Make DevTools a toggle in Settings > Developer tab

- DevTools off by default (no more auto-open on launch)
- New "Developer" tab in Settings with a checkbox to toggle devtools
- Toggle takes effect immediately (opens/closes inspector)
- Setting persists: devtools restored on next launch if enabled
- toggle_devtools Tauri command wraps window.open/close_devtools

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

src-tauri/src/commands/settings.rs

@@ -32,3 +32,16 @@ pub fn save_settings(settings: Value) -> Result<(), String> {
     fs::write(&path, json).map_err(|e| format!("Cannot write settings: {e}"))?;
     Ok(())
 }
+
+/// Toggle devtools on the main window.
+#[tauri::command]
+pub fn toggle_devtools(app: tauri::AppHandle, open: bool) {
+    use tauri::Manager;
+    if let Some(window) = app.get_webview_window("main") {
+        if open {
+            window.open_devtools();
+        } else {
+            window.close_devtools();
+        }
+    }
+}