v0.5.3 — tray binds → click to open in Finder

The Shared Folders submenu becomes useful at a glance. Each active
binding now appears at the top of the submenu as a click target —
clicking opens the bound local folder in Finder. No more "where
did I bind that to again?"

• Dynamic per-binding entries at the top of the Shared Folders
  submenu. One row per active binding in the format
  <bucket> ↔ <local-path> (home prefix compressed to ~/).
  Click opens the local folder in Finder. Sidecar JSONs at
  ~/.cache/garagetytus/bisync/*.bindings.json (written by
  garagetytus folder bind v0.5.3+) are the source of truth.
  Empty case shows a disabled "No folders bound yet" item so the
  user can tell the difference between "nothing bound" and
  "garagetytus not installed".
• New shared_folders::list_bindings(), Binding struct,
  menu_id_open_binding, parse_open_binding_id, and
  open_binding_in_finder helpers — all in
  tray/src/shared_folders.rs. 2 new unit tests (round-trip the
  ID + reject unrelated IDs); 17 tray tests total.