feat: link pages with mentions and keep images inside notes

[AmrMsCLL]

What changed

PR 3f from .claude/.Plans/as2026-10-02-strata-phase-3-technical-design.md. Stacked on #43.

New frontend dependencies: @tiptap/extension-mention, @tiptap/suggestion and @tiptap/extension-image, 3.31.4, all MIT and part of the Tiptap core Amr approved. Installed in the Linux container; the lockfile changes are the new entries, plus optional flags dropped on packages that are now direct dependencies.

• Mentions:
  • typing @ and two characters searches pages, tasks, events and anything else you can open, through the existing search;
  • arrow keys and Enter pick a result, Escape closes the menu;
  • a mention shows as @Title, and clicking it opens what it points to.
• Backlinks:
  • on every save, the server turns mentions into item_links of kind mention, removes the ones you deleted, and only links items the writer can read, so a mention can never reveal someone else's page;
  • each page shows "Linked from" for the pages and items that mention it.
• Images:
  • pasting or dropping an image uploads it to the note's space through the normal upload (shrunk and stripped as usual, 25 MB at most);
  • it shows inline, loaded with your session, because file downloads need it;
  • on save it becomes an attachment link from the note to the file;
  • an image whose file was deleted says so instead of showing a broken picture.
• The editor chunk grows from 169 kB to 188 kB gzipped, still loaded only when a page opens.

Verification

• Lint, both builds and tsc --noEmit pass.

• test/integration/notes.spec.ts adds a case covering:

  • a mention and an image becoming links, while a mention of someone else's private page is ignored;
  • the backlink showing on the target;
  • removing the mention removing its link.

  All five notes cases and records.spec.ts pass against Postgres on the local stack.

• test/notes.spec.tsx adds a case for "Linked from", which shows mentions only and links each item to its page.

• On the real local stack: typing "See @pack" in a new page listed the earlier page found by its text; Enter inserted the mention, and item_links got the mention row.

• Pasting an image wasn't tried in the browser, because it needs a real clipboard. The server side of attachments is covered by the integration test.