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/desktop/widgets/remote_toolbar.dart

@@ -5,6 +5,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_hbb/common/widgets/audio_input.dart';
 import 'package:flutter_hbb/common/widgets/dialog.dart';
+import 'package:flutter_hbb/common/widgets/keyboard_shortcuts/display.dart';
 import 'package:flutter_hbb/common/widgets/toolbar.dart';
 import 'package:flutter_hbb/models/chat_model.dart';
 import 'package:flutter_hbb/models/state_model.dart';
@@ -763,8 +764,31 @@ class _ControlMenu extends StatelessWidget {
               if (e.divider) {
                 return Divider();
               } else {
+                final hint = e.actionId == null
+                    ? null
+                    : ShortcutDisplay.formatFor(e.actionId!);
+                final child = hint == null
+                    ? e.child
+                    : Row(
+                        mainAxisSize: MainAxisSize.min,
+                        children: [
+                          Flexible(child: e.child),
+                          Padding(
+                            padding: const EdgeInsets.only(left: 16),
+                            child: Text(
+                              hint,
+                              style: Theme.of(context)
+                                  .textTheme
+                                  .bodySmall
+                                  ?.copyWith(
+                                    color: Theme.of(context).hintColor,
+                                  ),
+                            ),
+                          ),
+                        ],
+                      );
                 return MenuButton(
-                    child: e.child,
+                    child: child,
                     onPressed: e.onPressed,
                     ffi: ffi,
                     trailingIcon: e.trailingIcon);