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/consts.dart

@@ -686,3 +686,24 @@ extension WindowsTargetExt on int {
 }
 
 const kCheckSoftwareUpdateFinish = 'check_software_update_finish';
+
+// Keyboard shortcut Action IDs - must match src/keyboard/shortcuts.rs::action_id.
+const kShortcutActionSendCtrlAltDel = 'send_ctrl_alt_del';
+const kShortcutActionToggleFullscreen = 'toggle_fullscreen';
+const kShortcutActionSwitchDisplayNext = 'switch_display_next';
+const kShortcutActionSwitchDisplayPrev = 'switch_display_prev';
+const kShortcutActionScreenshot = 'screenshot';
+const kShortcutActionInsertLock = 'insert_lock';
+const kShortcutActionRefresh = 'refresh';
+const kShortcutActionToggleAudio = 'toggle_audio';
+const kShortcutActionToggleBlockInput = 'toggle_block_input';
+const kShortcutActionToggleRecording = 'toggle_recording';
+const kShortcutActionTogglePrivacyMode = 'toggle_privacy_mode';
+const kShortcutActionViewMode1to1 = 'view_mode_1_to_1';
+const kShortcutActionViewModeShrink = 'view_mode_shrink';
+const kShortcutActionViewModeStretch = 'view_mode_stretch';
+const kShortcutActionSwitchSides = 'switch_sides';
+String kShortcutActionSwitchTab(int n) => 'switch_tab_$n';
+
+const kShortcutLocalConfigKey = 'keyboard-shortcuts';
+const kShortcutEventName = 'shortcut_triggered';