Extension popup: mode choice, latest report, panel link; auto-copied share links ...

- Toolbar click now opens a popup instead of jumping straight into the
  picker: two mode buttons (element note / record steps with live step
  count), the latest report of the default project with open and
  two-step delete, and an "Open web panel" shortcut to the project page.
- New background messages (start_picker, toggle_recorder, latest_report,
  delete_report, open_panel) resolve the active tab themselves; the
  action.onClicked path stays for keyboard-driven flows.
- Settings gain panelUrl (falls back to serverUrl); options page gets a
  "Web panel URL" input.
- After a note is submitted its share link is copied to the clipboard
  automatically (with an execCommand fallback) and a brief toast
  confirms it; e2e asserts the clipboard content.
- Build: new popup entry (own CSS chunk), manifest default_popup.

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