Make archived documents read plainly as archived

Rename the sidebar archive controls to "Archived" and "All documents". An
archived document's header now shows a neutral Archived badge with a Restore
button, the chat composer is replaced by a quiet notice, and an empty
read-only document says it is empty instead of inviting writing. Viewers
without write access get the same read-only composer notice.

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