feat(command-line): copy/clear history + selectable log (#232)

The command-line history dropdown was display-only, so users had to
screenshot and OCR the log to report output. Make it copyable:

- Render the whole backlog in one read-only text_editor so a single
  mouse drag selects across lines and Ctrl+C copies the span.
- Add Copy (whole log to clipboard) and Clear buttons to the dropdown.
- Gate the Ctrl+C/X/V accelerators on event Status::Ignored so a focused
  text widget's copy/cut/paste wins instead of firing COPYCLIP/CUTCLIP;
  the drawing's clipboard shortcuts still work when it has focus.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/icons/ui/copy.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#000000" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round"><rect x="9" y="9" width="11" height="11" rx="2"/><path d="M5 15 H4 a1 1 0 0 1 -1 -1 V4 a1 1 0 0 1 1 -1 H14 a1 1 0 0 1 1 1 V5"/></svg>