- Add CollaboratorWebSocket handler for real-time collaboration
- Add CollaborationHub field to Handler struct
- Add validateWSToken and colorFromDID helper functions
- Add RegisterClient/UnregisterClient methods to Room
- Wire up Hub in main.go and register /ws/docs/{rkey} route

Add SVG favicons and PNG variants (16, 32, 48, 180, 192, 512px) with
font-style:normal to ensure upright rendering of the Dd logotype.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Add @milkdown/plugin-history to the Milkdown editor, enabling undo/redo
via Mod-z/Mod-Shift-z keyboard shortcuts. Also adds ↩/↪ toolbar buttons
visible only in rich mode.

- Add logo-document.svg and logo-monogram.svg to static/img
- Show document logo in navbar alongside Diffdown wordmark
- Add Documents link to navbar (left of New Document) for logged-in users
- Remove Documents button from document view actions
- Add btn-danger style: red hover for outline delete button
- Fix list-view card hover highlight using background tint instead of border

Documents are now stored as com.diffdown.document records instead of
site.standard.document, keeping Diffdown docs private from Standard
aggregators. Removes the publication concept entirely (no ensurePublication,
no site field). Renames publishedAt → createdAt. Keeps at.markpub.markdown
as the content type for interop.

- Click a link in rich text mode to show an inline tooltip with URL input
- Tooltip supports update, remove, and cancel actions
- Editor rich container now uses --bg for contrast; milkdown pane styled as a document page with border, shadow, and padding
- Increase paragraph spacing (1.25em) and reduce list item spacing (0.25em)
- Fix p > li margin bleed in both editor and markdown-body views