feat: keyboard shortcuts in remote sessions

Add an opt-in keyboard-shortcut system that triggers session
actions (Send Ctrl+Alt+Del, Toggle Fullscreen, Switch Display,
Screenshot, Switch Tab, etc.) via three-modifier combinations
during a remote session.

Architecture
- Native: src/keyboard/shortcuts.rs intercepts at the encoder
  layer (process_event and process_event_with_session), so the
  feature is input-source-independent. Bindings persist as a
  single JSON blob in LocalConfig.
- Web: matching + keydown intercept live in the separate hand-
  written TS client at flutter/web/js/ (gitignored, not in this
  repo). flutter/lib/web/bridge.dart::mainInit registers
  window.onShortcutTriggered so the JS matcher can dispatch
  back into the active session's ShortcutModel; the bridge's
  mainReloadKeyboardShortcuts forwards to a JS reloadShortcuts
  on settings writes.
- Three-modifier prefix (Ctrl+Alt+Shift; Cmd+Option+Shift on
  macOS/iOS) sidesteps the need for a pass-through toggle.
- Flutter native path threads the explicit per-call SessionID
  for tab-precise routing; rdev path uses globally-current
  session.

UI
- Settings -> General -> Keyboard Shortcuts opens a dedicated
  configuration page; desktop and mobile share a body widget.
- Recording dialog with live capture, prefix validation, and a
  conflict-replace flow.
- Toolbar menu items display the bound shortcut inline.
- Default bindings (adapted from AnyDesk):
    +Del    Send Ctrl+Alt+Del
    +Enter  Toggle Fullscreen
    +Left/Right  Switch Display Prev/Next
    +P      Screenshot
    +1..9   Switch Session Tab

Other
- AGENTS.md: documented (a) flutter_rust_bridge_codegen needs
  a pinned version + Dart bridge wrappers should be hand-
  written, and (b) the Web-target split where flutter/web/js/
  is the runtime owner on Web rather than wasm-compiled Rust.
- 38 new i18n strings in src/lang/en.rs with Chinese
  translations in src/lang/cn.rs.

Refs discussion #1933.

flutter/lib/mobile/pages/remote_page.dart

@@ -21,6 +21,7 @@ import '../../common/widgets/remote_input.dart';
 import '../../models/input_model.dart';
 import '../../models/model.dart';
 import '../../models/platform_model.dart';
+import '../../models/shortcut_model.dart';
 import '../../utils/image.dart';
 import '../widgets/dialog.dart';
 import '../widgets/custom_scale_widget.dart';
@@ -119,6 +120,18 @@ class _RemotePageState extends State<RemotePage> with WidgetsBindingObserver {
       }
       _disableAndroidSoftKeyboard(
           isKeyboardVisible: keyboardVisibilityController.isVisible);
+      // Seed shortcut action callbacks once the session is ready, so that
+      // global keyboard shortcuts work even if the user never opens the
+      // toolbar menu. The returned list is intentionally discarded — the
+      // side effect of registering callbacks (inside toolbarControls) is
+      // what we want here.
+      if (mounted) {
+        toolbarControls(context, widget.id, gFFI);
+        // Mobile has no DesktopTabController, so tab-switch shortcuts
+        // remain unregistered (they will simply log a no-handler debug
+        // line if a mobile user binds one — they have no tabs to switch).
+        registerSessionShortcutActions(gFFI);
+      }
     });
     WidgetsBinding.instance.addObserver(this);
   }