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/models/model.dart

@@ -21,6 +21,7 @@ import 'package:flutter_hbb/models/peer_model.dart';
 import 'package:flutter_hbb/models/peer_tab_model.dart';
 import 'package:flutter_hbb/models/printer_model.dart';
 import 'package:flutter_hbb/models/server_model.dart';
+import 'package:flutter_hbb/models/shortcut_model.dart';
 import 'package:flutter_hbb/models/user_model.dart';
 import 'package:flutter_hbb/models/state_model.dart';
 import 'package:flutter_hbb/models/desktop_render_texture.dart';
@@ -476,6 +477,11 @@ class FfiModel with ChangeNotifier {
       } else if (name == 'exit_relative_mouse_mode') {
         // Handle exit shortcut from rdev grab loop (Ctrl+Alt on Win/Linux, Cmd+G on macOS)
         parent.target?.inputModel.exitRelativeMouseModeWithKeyRelease();
+      } else if (name == kShortcutEventName) {
+        final action = evt['action'];
+        if (action is String) {
+          parent.target?.shortcutModel.onTriggered(action);
+        }
       } else {
         debugPrint('Event is not handled in the fixed branch: $name');
       }
@@ -3623,6 +3629,7 @@ class FFI {
   late final ElevationModel elevationModel; // session
   late final CmFileModel cmFileModel; // cm
   late final TextureModel textureModel; //session
+  late final ShortcutModel shortcutModel; // session
   late final Peers recentPeersModel; // global
   late final Peers favoritePeersModel; // global
   late final Peers lanPeersModel; // global
@@ -3652,6 +3659,7 @@ class FFI {
     elevationModel = ElevationModel(WeakReference(this));
     cmFileModel = CmFileModel(WeakReference(this));
     textureModel = TextureModel(WeakReference(this));
+    shortcutModel = ShortcutModel(WeakReference(this));
     recentPeersModel = Peers(
         name: PeersModelName.recent,
         loadEvent: LoadEvent.recent,