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.

src/lang/cn.rs

@@ -743,5 +743,44 @@ pub static ref T: std::collections::HashMap<&'static str, &'static str> =
         ("Display Name", "显示名称"),
         ("password-hidden-tip", "永久密码已设置（已隐藏）"),
         ("preset-password-in-use-tip", "当前使用预设密码"),
+        ("Keyboard Shortcuts", "键盘快捷键"),
+        ("Configure shortcuts...", "配置快捷键..."),
+        ("Enable keyboard shortcuts in remote session", "在远程会话中启用键盘快捷键"),
+        ("shortcut-page-description", "启用后，列出的组合键将在本地触发会话操作，而不会发送到远程端。所有快捷键必须包含 Ctrl+Alt+Shift（macOS 上为 Cmd+Option+Shift），以避免与正常输入冲突。"),
+        ("Reset to defaults", "恢复默认设置"),
+        ("shortcut-reset-confirm-tip", "这将以默认快捷键替换所有当前绑定。是否继续？"),
+        ("Session Control", "会话控制"),
+        ("Toggle Fullscreen", "切换全屏"),
+        ("Switch to next display", "切换到下一个显示器"),
+        ("Switch to previous display", "切换到上一个显示器"),
+        ("View Mode 1:1", "原始大小"),
+        ("View Mode Shrink", "缩小"),
+        ("View Mode Stretch", "拉伸"),
+        ("Take Screenshot", "截图"),
+        ("Toggle Audio", "切换音频"),
+        ("Toggle Privacy Mode", "切换隐私模式"),
+        ("Toggle Recording", "切换录制"),
+        ("Toggle Block User Input", "切换屏蔽用户输入"),
+        ("Switch Tab 1", "切换到第 1 个标签"),
+        ("Switch Tab 2", "切换到第 2 个标签"),
+        ("Switch Tab 3", "切换到第 3 个标签"),
+        ("Switch Tab 4", "切换到第 4 个标签"),
+        ("Switch Tab 5", "切换到第 5 个标签"),
+        ("Switch Tab 6", "切换到第 6 个标签"),
+        ("Switch Tab 7", "切换到第 7 个标签"),
+        ("Switch Tab 8", "切换到第 8 个标签"),
+        ("Switch Tab 9", "切换到第 9 个标签"),
+        ("Edit", "编辑"),
+        ("Save", "保存"),
+        ("Set Shortcut", "设置快捷键"),
+        ("shortcut-recording-instruction", "请按下您想使用的组合键。"),
+        ("shortcut-recording-press-keys-tip", "请按下组合键..."),
+        ("shortcut-must-include-prefix", "必须包含"),
+        ("shortcut-already-bound-to", "已绑定到"),
+        ("Replace", "替换"),
+        ("Valid", "有效"),
+        ("shortcut-mobile-physical-keyboard-tip", "录制需要使用物理键盘，不支持软键盘。"),
+        ("On", "开"),
+        ("Off", "关"),
     ].iter().cloned().collect();
 }