fix(ui): keep an unnamed class out of the schema removal preview, one at a time

[JArmandoAnaya]

What changed

Deleting a class in the Schema tab previews the remaining draft through POST /schema/preview. That body is publish-shaped, and the kernel refuses a blank class name, so with two or more unnamed classes every delete surfaced as "Could not preview this change · The request payload is not processable". The kernel and the wire are right and untouched; the editor was sending an unpublishable document to a publish-shaped endpoint.

• SchemaEditor.tsx: a class with no name was never published and cannot carry annotations, so removing it is a local edit with no request; removing a named class previews the named classes only.
• Add class adds nothing while a class is still unnamed — it selects that class, clears the filter, and says "Name the new class first", the remedy Save already uses for the same blank. Typing a name unblocks it; no save is required. The gate is on the name only, because a new class always carries box and the picker never lets the last shape be unticked.
• docs/content/ui.md: the schema-editor section states the rule.

Found, not fixed

Nothing.

Test plan

• vitest on screens.test.tsx + schemaDraft.test.tsx: 129 pass. Three new cases — blank removal sends no preview, named removal sends only the named classes, second Add is refused until the first is named — each proven to fail with the fix stashed.
• pnpm --filter @visionset/ui-core lint
• CI: Playwright suites and the rest of the matrix