plugins: no third-party library ships a Pangular tab yet #404

The public plugin API shipped in #399, but no outside library uses it yet. This was the last "done when" goal of #343, moved here so the phase 4 issue can close.

Done when

• At least one third-party library (for example a Signal Store add-on, TanStack Query or Spartan) ships its own Pangular tab through the plugin API.

Refs #343.