Heading selector now updates as you move the cursor

The heading selector in the editor toolbar used to show whatever heading level you had most recently clicked, even after you moved the cursor into a different block. So you could be editing an H2 with the dropdown stuck on "Paragraph".

The selector now reactively tracks your cursor — move into an H2, the dropdown reads H2; move into a paragraph, it reads Paragraph. The label only changes when the underlying value actually changes, so there's no needless re-rendering.

Small thing, but in a heavy editing session the constant state drift was just enough friction to be annoying. Gone now.