docs: require Vue views, Pinia stores and the kit for new UI

CONTRIBUTING gains a Building UI section: new UX elements are Vue
components rendering a Pinia store their owner publishes to, portable
owners stay framework-free behind a publish option, controls come from
src/ui/kit first, controllers don't bind the DOM by hand (Teleport where a
view renders elsewhere; body classes, focus, same-task measured positions
and painted surfaces stay imperative), text goes through t()/tn() with
msg() for stored English, and tests mount components. The README and the
architecture summary drop the stale vanilla-JS claim, the UI ownership doc
points to the guide, and the migration doc's UiPanel note names
v-panel-state as what draws a panel's collapse.

docs/UI-OWNERSHIP.md

@@ -18,6 +18,11 @@ scene playback and voice. It delegates; new behavior belongs with its owner.
 | `shareRestoration.js` | Initial share/layer restoration and cancellation |
 | `shellFeedback.js` | Loading feedback, notices and toast lifetime |
 
+Owners that present UI do so through Vue: they publish plain view objects to a
+Pinia store in `src/ui/stores/`, views in `src/ui/views/` render them, and user
+actions return through the handlers the owner connects to that store. See
+[Building UI](../CONTRIBUTING.md#building-ui).
+
 Owners receive named services, elements, operations and readers. They do not
 receive the application shell itself. Readers resolve a replaceable collaborator
 at use time; operations cross an ownership boundary without copying its state.