Add mouse selection and editing to the fullscreen composer - #46858

## What changed

- Support click-to-position, drag selection, double-click word selection, and triple-click logical line selection in the composer, respecting wrapping, scrolling, and atomic grapheme and placeholder boundaries.
- Copy selected draft text through the existing clipboard status flow without changing the draft or selection, before interrupt and exit shortcuts consume the key.
- Replace or delete selected text with editor input, flush buffered typing before mouse edits, and reconcile removed paste and image placeholders.
- Coordinate composer gestures with transcript selections, completion popups, and modals. Use generic retry messages for copy failures.

## Testing

Add coverage for Unicode and wrapped-text hit testing, selection highlighting and editing, multiclick dragging, placeholder cleanup, history navigation, and fullscreen copy and input ownership.

GitOrigin-RevId: 939ef461ec67855b3e87c3c7ddd8a60a3250c7cd