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/settings_page.dart

@@ -17,8 +17,10 @@ import '../../common/widgets/login.dart';
 import '../../consts.dart';
 import '../../models/model.dart';
 import '../../models/platform_model.dart';
+import '../../models/shortcut_model.dart';
 import '../widgets/dialog.dart';
 import 'home_page.dart';
+import 'mobile_keyboard_shortcuts_page.dart';
 import 'scan_page.dart';
 
 class SettingsPage extends StatefulWidget implements PageShape {
@@ -819,6 +821,22 @@ class _SettingsState extends State<SettingsPage> with WidgetsBindingObserver {
               showThemeSettings(gFFI.dialogManager);
             },
           ),
+          SettingsTile.navigation(
+            leading: Icon(Icons.keyboard_outlined),
+            title: Text(translate('Keyboard Shortcuts')),
+            description: Text(ShortcutModel.isEnabled()
+                ? translate('On')
+                : translate('Off')),
+            onPressed: (context) {
+              Navigator.push(
+                  context,
+                  MaterialPageRoute(
+                    builder: (_) => const MobileKeyboardShortcutsPage(),
+                  )).then((_) {
+                if (mounted) setState(() {});
+              });
+            },
+          ),
           if (!bind.isDisableAccount())
             SettingsTile.switchTile(
               title: Text(translate('note-at-conn-end-tip')),
@@ -1352,3 +1370,4 @@ SettingsTile _getPopupDialogRadioEntry({
     ),
   );
 }
+