Selection copy:
- Route mouse motion and release to the chat while a text-selection
  drag is active, so a drag that ends over the editor or status bar
  still extends and finalizes the selection instead of dropping it.
- Treat triple-click like double-click: anchor the line, let the drag
  extend it, and copy on release or via the debounce, instead of
  copying the line immediately on press and ignoring the drag.
- Fall back to a plain drag selection when a double or triple click
  lands on a blank line instead of leaving a dead drag.
- Reset multi-click tracking after a completed drag and cycle the
  click count after a triple-click, so quick selection retries are
  not misdetected as word or line selections.
- Preserve code indentation (dedent to the common prefix), trim the
  right-hand width padding, and drop UI affordance lines (copy, edit,
  retry labels, expand and collapse indicators) and boundary blank
  lines from copied text.
- Fix findClosestRuneIndex dropping the last character when the
  selection extends past the end of an unpadded line.

Copy affordances:
- Widen the copy labels from "⎘" to "⎘ copy" on assistant messages
  and code blocks for a larger click target.
- Flash a same-width "copied" confirmation in place of the clicked
  label for 1.5s.

Adds a tuitest Drag helper and an e2e test that releases the drag
over the editor region, which fails without the routing fix.