feat: embed boards in pages with a live preview

[AmrMsCLL]

Boards step B2.

• Embed a board in a page: /board in the page editor creates a board inside the page and embeds it. The embed shows a preview of the drawing (Excalidraw's exportToSvg, loaded only when a preview is on screen, images included) and an Open board link. Embedded boards count as links, so the board lists the page under Linked from.
• Preview data: GET /notes/:id/board returns a board's current elements from storage (boards only; pages get 404).
• Board menu: Excalidraw's menu is replaced with Export image (PNG/SVG), Save to file, Find on canvas, the canvas colour and Help. Its GitHub/Discord links and Reset the canvas (which would wipe a shared board) are gone.
• The Excalidraw asset path and image loading moved to components/board-files.ts, shared by the canvas and the preview.
• The page format goes to 3, so older tabs reload instead of dropping embeds.

Checked in the browser (local stack): /board adds an embed; after drawing on the board the page shows the preview; an image dropped on the board uploads through /spaces/:id/files, is stored on the board by file id and appears in the preview; the board menu shows only the items above.

Tests: test/block-menu.spec.tsx (the Board entry), test/notes.spec.tsx, test/document-text.spec.ts (embeds become links); test/integration/notes.spec.ts covers the board endpoint (CI).