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/common/widgets/keyboard_shortcuts/display.dart

@@ -0,0 +1,65 @@
+// flutter/lib/common/widgets/keyboard_shortcuts/display.dart
+import 'dart:convert';
+import 'package:flutter/foundation.dart';
+import '../../../consts.dart';
+import '../../../models/platform_model.dart';
+
+/// Read the bindings JSON and produce a human-readable shortcut string for
+/// `actionId`, formatted for the current OS. Returns null if unbound.
+class ShortcutDisplay {
+  static String? formatFor(String actionId) {
+    final raw = bind.mainGetLocalOption(key: kShortcutLocalConfigKey);
+    if (raw.isEmpty) return null;
+    final Map<String, dynamic> parsed;
+    try {
+      parsed = jsonDecode(raw) as Map<String, dynamic>;
+    } catch (_) {
+      return null;
+    }
+    if (parsed['enabled'] != true) return null;
+    final list = (parsed['bindings'] as List? ?? []).cast<Map<String, dynamic>>();
+    final found = list.firstWhere(
+      (b) => b['action'] == actionId,
+      orElse: () => {},
+    );
+    if (found.isEmpty) return null;
+
+    // Guard against a hand-edited / corrupt config where `key` is missing or
+    // not a string — silently treat the binding as unbound rather than
+    // crashing the toolbar render.
+    final keyValue = found['key'];
+    if (keyValue is! String) return null;
+
+    final isMac = defaultTargetPlatform == TargetPlatform.macOS ||
+                  defaultTargetPlatform == TargetPlatform.iOS;
+    // `mods` similarly may be malformed; treat a non-list as no modifiers.
+    final modsRaw = found['mods'];
+    final mods = modsRaw is List
+        ? modsRaw.whereType<String>().toList()
+        : const <String>[];
+    final parts = <String>[];
+    for (final m in ['primary', 'alt', 'shift']) {
+      if (!mods.contains(m)) continue;
+      switch (m) {
+        case 'primary': parts.add(isMac ? '⌘' : 'Ctrl'); break;
+        case 'alt':     parts.add(isMac ? '⌥' : 'Alt'); break;
+        case 'shift':   parts.add(isMac ? '⇧' : 'Shift'); break;
+      }
+    }
+    parts.add(_keyDisplay(keyValue, isMac));
+    return isMac ? parts.join('') : parts.join('+');
+  }
+
+  static String _keyDisplay(String key, bool isMac) {
+    switch (key) {
+      case 'delete':     return isMac ? '⌫' : 'Del';
+      case 'enter':      return isMac ? '⏎' : 'Enter';
+      case 'arrow_left': return '←';
+      case 'arrow_right':return '→';
+      case 'arrow_up':   return '↑';
+      case 'arrow_down': return '↓';
+    }
+    if (key.startsWith('digit')) return key.substring(5);
+    return key.toUpperCase();
+  }
+}