v0.3.0

Live monitoring and file operations

Live monitoring
- Search results now update as files change under a monitored folder:
  a file created, copied in from a USB drive or network share, or deleted
  externally shows up (or disappears) within a second or two, no manual
  "Rebuild Index Now". Bursts (a large copy) coalesce into one refresh.
- A directory that appears already populated -- mkdir d && touch d/f, or
  moving a populated folder in -- no longer loses its contents to the
  window between the directory existing and inotify watching it.

File operations (docs/adr/0013, supersedes 0005)
- Ctrl+C copies the file object: pasting into Nautilus, Nemo, Caja, or
  Thunar copies the actual file. Plain-text path still available for text
  editors. Ctrl+X is the move-on-paste counterpart.
- Dragging a result row out to a file manager now works.
- Shift+Delete permanently deletes the selection behind a confirm dialog;
  plain Delete still moves to Trash.
- Right-click Copy and Cut entries in the result context menu.
- Trash and permanent-delete now report failures instead of silently
  leaving the index inconsistent.

Also: the version string, stuck at 0.1.0 through the v0.1.1 and v0.2.0
tags, is synced to 0.3.0.

Full test suite green; clang-format and cppcheck clean; changes covered
by two independent code reviews.