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>

src/web/public/app.js

@@ -349,6 +349,22 @@ const DEFAULT_SHORTCUTS = [
     bindings: [{ modifiers: ['ctrl'], key: 'l' }],
     action: 'clearTerminal',
   },
+  {
+    id: 'copy-selection',
+    group: 'Terminal',
+    label: 'Copy Selection',
+    // Bindings match on `key`, not `code`: xterm decides which byte to emit from the
+    // PRODUCED character, so intercepting a physical KeyC that doesn't produce "c"
+    // would diverge from the chord that actually sends ^C.
+    bindings: [
+      { modifiers: ['ctrl'], key: 'c' },
+      { modifiers: ['ctrl', 'shift'], key: 'C' },
+    ],
+    // Dispatched by shouldCopyTerminalSelectionFromShortcut() in terminal-ui.js and
+    // deliberately absent from SHORTCUT_ACTIONS: the generic capture loop always
+    // preventDefaults on a match, which would cost the user the interrupt key.
+    action: 'copyTerminalSelection',
+  },
   {
     id: 'increase-font',
     group: 'Terminal',