feat(terminal): Ctrl+C copies the selection, interrupts when nothing is selected

Closes #211. Copying from the terminal only worked through the browser
context menu, because xterm turns Ctrl+C into 0x03 and cancels the keydown,
so the muscle-memory copy failed silently and read as "no copy-paste at all".

With a selection, Ctrl+C now copies it, toasts, clears the selection and
sends nothing to the PTY. With no selection it falls through unchanged, so
the interrupt is intact. Ctrl+Shift+C is an explicit copy chord that never
falls through: an explicit copy that interrupts a running agent because the
selection happened to be empty would be a footgun.

Three details that keep the interrupt safe:

- The decision lives in attachCustomKeyEventHandler (terminal-ui.js) and the
  no-selection path returns true WITHOUT preventDefault. xterm calls the
  custom handler before its own cancel(), so returning false alone does not
  cancel the event; the copy path therefore calls preventDefault explicitly,
  or the browser would run its native copy on top of ours.
- copy-selection is a full registry entry (rebindable and disableable in App
  Settings) whose action is deliberately absent from SHORTCUT_ACTIONS, the
  same trick command-palette uses: the generic capture loop preventDefaults
  every match it dispatches, which would cost the user the interrupt key.
- The gate is keydown-only, since the custom handler also runs for keypress
  and keyup.

Copy goes through _copyText (Clipboard API, then hidden-textarea +
execCommand) rather than raw navigator.clipboard, because install.sh's LAN
option serves plain HTTP where navigator.clipboard is undefined; the
fallback steals focus, so the terminal is refocused afterwards.

Tests: test/terminal-copy-selection.test.ts pins the gate and the
SHORTCUT_ACTIONS invariant; test/terminal-copy-shortcut.test.ts drives real
key presses in chromium and asserts on the clipboard plus the bytes xterm
emitted (browser-driven, so excluded from test:ci like the other Playwright
suites). Verified manually on an isolated beta instance before landing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.zh-CN.md

@@ -641,6 +641,8 @@ sc -l           # 列出会话
 | `Alt/Option+[` / `Alt/Option+]` | 上一个 / 下一个会话                                      |
 | `Alt/Option+1`–`Alt/Option+9`   | 切换到第 N 个标签（按物理键位，macOS Option 布局也适用） |
 | `Ctrl+Shift+{` / `Ctrl+Shift+}` | 将当前标签左移 / 右移                                    |
+| `Ctrl/Cmd+C`                    | 复制选中内容；未选中时中断代理                           |
+| `Ctrl+Shift+C`                  | 复制选中内容（永不中断）                                 |
 | `Ctrl/Cmd+L`                    | 清屏                                                     |
 | `Ctrl+Shift+R`                  | 恢复终端尺寸                                             |
 | `Ctrl+Shift+V`                  | 切换语音输入                                             |