feat(terminal): Auto Copy, put a finished selection on the clipboard

App Settings > Terminal & Input > Selection & clipboard > Auto Copy
Selection (`autoCopySelection`, per-device, default OFF). With it on,
highlighting text in the terminal copies it: mouse drag, double-click
word, triple-click line, and the phone long-press selection. Ctrl+C is
untouched and still copies on demand.

Three things decide the shape of it:

* It fires at the END of a gesture, never in onSelectionChange. That
  callback runs for every cell a drag crosses, so copying there would be
  one clipboard write per mouse move. It only arms a pending flag; a
  document-level mouseup listener flushes, and the touch path calls the
  flush itself because it preventDefaults its touchend and no mouseup
  ever arrives there.
* The flush is synchronous inside the handler, because both clipboard
  paths need user activation: Firefox gates navigator.clipboard
  .writeText on it, and execCommand('copy'), the fallback the plain-HTTP
  LAN install lands on, has to run in the gesture's own task. A timer or
  a wait for onSelectionChange loses it, invisibly in Chrome.
* It deliberately does NOT do what copyTerminalSelection() does. That
  one clears the selection (so a second Ctrl+C is an interrupt) and
  focuses the terminal. Clearing would make text vanish under the cursor
  that just highlighted it, and focusing opens the on-screen keyboard
  over it on a phone. Focus is instead restored to whatever held it,
  which only matters for the execCommand fallback.

Guards are pure in decideAutoCopy() (constants.js): off, blank or
whitespace-only text, and a 1M-char cap, since a drag off the top of the
viewport autoscrolls and one gesture can sweep the whole 50k-line
scrollback. Past the cap the copy is refused rather than truncated, with
a toast pointing at Ctrl+C.

Feedback is silent on success except once per page load, so a feature
that works by doing nothing visible can still be told from a dead
toggle; failures and refusals toast, throttled to 10s.

Per-device on both counts the settings rule requires: in `displayKeys`
and absent from the .strict() SettingsUpdateSchema, because clipboard
access differs by device and by origin.

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

src/web/public/i18n.js

@@ -335,6 +335,11 @@
     'Extended Keyboard Bar': '扩展键盘栏',
     'Gesture Control (beta)': '手势控制（测试版）',
     'Wheel Scrolls Local History': '滚轮滚动本地历史',
+    'Auto Copy Selection': '自动复制选中内容',
+    'Selection & clipboard': '选中与剪贴板',
+    'Auto Copy: selection copied': '自动复制：已复制选中内容',
+    'Auto Copy failed: the browser blocked clipboard access': '自动复制失败：浏览器阻止了剪贴板访问',
+    'Selection too large to copy automatically. Press Ctrl+C.': '选中内容过大，无法自动复制。请按 Ctrl+C。',
     'Instant typing feedback with local echo': '通过本地回显即时显示输入',
     'Dedicated IME input field for CJK languages': '为中日韩语言提供专用输入法文本框',
     'Extra keys: Tab, Esc, arrows, Ctrl+O': '附加按键：Tab、Esc、方向键、Ctrl+O',