Edit the document title in place in the header

Clicking the header title, pressing F2, or choosing Rename from the
document menu now swaps the title for an inline field. Enter or blur
commits through the existing rename request; Escape reverts. A document
created from New document opens with its generated name selected, and
committing or dismissing it hands the caret to the document body.
Archived and read-only viewers keep plain text.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>