feat: add the notes editor with live co-editing

[AmrMsCLL]

What changed

PR 3d from .claude/.Plans/as2026-10-02-strata-phase-3-technical-design.md. Amr approved decision 3, the Tiptap core. Stacked on #41.

New frontend dependencies, all MIT and from the free Tiptap core, never its paid cloud: @tiptap/core, react, starter-kit, pm, extension-collaboration, extension-collaboration-caret, extension-list (checklists) and extensions (placeholder), all 3.31.4; plus yjs 13.6.33 and @hocuspocus/provider 4.7.0. They were installed in the Linux container so the lockfile keeps its libc fields.

• A Notes tab in every space:
  • pages are listed as an indented tree, and "New page" opens a fresh page;
  • pages whose parent is gone show at the top level rather than disappearing.
• /notes/:id:
  • a breadcrumb back through the space and parent pages;
  • a title you edit in place, saved when you leave the field or press Enter;
  • the editor;
  • "Pages inside", with "Add a page inside".
• The editor:
  • Tiptap with bold, italic, two heading levels, bulleted, numbered and check lists, quotes and code blocks, and Markdown shortcuts as you type;
  • edits sync live through the real-time server from feat: add notes and a real-time editing server #41;
  • other people's cursors show with their names in a colour of their own, and their avatars appear under "Also here";
  • a status reads Connecting, Saving, Saved, Offline, or View only;
  • viewers get a read-only editor and title;
  • a note over the 5 MB limit shows an error toast;
  • the editor is loaded only when a page opens: 169 kB gzipped in its own chunk.
• Behind the scenes:
  • the connection sends the current access token and keeps working across the 13-minute renewal, without reconnecting;
  • search results and item lists now link to the page they belong to (notes, events, tasks, lists, subscriptions) through one itemHref helper;
  • the local Vite proxy now forwards WebSockets (ws: true).

Verification

• Lint, tsc -b and vite build pass.
• The new test/notes.spec.tsx replaces the editor with a stub. It covers:
  • tree order and depth;
  • listing and creating pages;
  • the breadcrumb, renaming on blur and adding a page inside;
  • a viewer's read-only page;
  • "Page not found".
• routes, spaces and files pass. In search, a note now opens /notes/:id instead of its space, so that one expectation changed.
• On the real local stack (Postgres, Redis and the real-time server):
  • creating a page from the Notes tab;
  • typing and seeing "Saved";
  • the text stored in note_documents and searchable;
  • reloading in a second tab, which brings the text back;
  • a live cursor from the other tab, and "Also here: Tester" in the first.