feat(shell): add Ctrl+C button to mobile terminal shortcuts

Add a Ctrl+C key to the right end of the mobile shortcuts bar so users
can interrupt the foreground process without a physical keyboard. Sends
\x03 (ETX), which is the same interrupt sequence on Windows and Linux.

src/components/shell/view/subcomponents/TerminalShortcutsPanel.tsx

@@ -26,6 +26,7 @@ const MOBILE_KEYS: Shortcut[] = [
   { type: 'arrow', id: 'arrow-down', sequence: '\x1b[B', icon: 'down' },
   { type: 'arrow', id: 'arrow-left', sequence: '\x1b[D', icon: 'left' },
   { type: 'arrow', id: 'arrow-right', sequence: '\x1b[C', icon: 'right' },
+  { type: 'key', id: 'ctrl-c', label: 'Ctrl+C', sequence: '\x03' },
 ];
 
 const ARROW_ICONS = {