Add a Whiteboard tab for free-form project notes

Goal

Give every project a Whiteboard: a free-form place to jot down notes and ideas about the project, a bit like a mini wiki. It works like the Context tab (add a page, write in it, delete it easily) but is lighter. Whiteboard pages are for general scratch notes that come and go, and they are never linked to issues.

Expected behavior

• A new Whiteboard project tab, next to Context.
• The Whiteboard shows a list of pages and the selected page's content, laid out like the Context tab.
• Creating a page takes a title and optional Markdown body, with one click to start.
• Editing uses the same explicit edit / save / cancel Markdown behavior as Context pages.
• Deleting a page is quick: one action with a lightweight confirmation.
• There's no issue linking anywhere. Whiteboard pages can't be attached to issues, and there's no linked-issues section.

Acceptance criteria

• Whiteboard pages are stored per project (new table and migration), separate from project context. They're soft-deleted like other project content, and deleted pages never show up.
• The tab has its own addressable URL, e.g. /{owner}/projects/{key}/whiteboard, plus a URL for each selected page.
• Pages are listed most recently updated first. The list stays compact and shows only titles; the body appears only for the selected page.
• Markdown renders the same way as Context pages, with the same external-image protection (TRACK-12).
• Permissions:
  • Project writers can create, edit and delete pages.
  • Read-only members and public viewers can read pages but see no edit controls (TRACK-19).
  • Anyone without project access can't see them.
• API and MCP parity: pages can be listed, read, created, updated and deleted through the REST API and MCP tools, under the same permission rules.
• Changes are recorded in the project changelog and pushed to other open clients via realtime events.
• Tab placement follows the existing project tab rules in DESIGN_CONTEXT.md:
  • On wide layouts Whiteboard is a tab.
  • On narrow layouts it moves into the project actions menu alongside Context and About.
  • The tab bar never wraps or scrolls.
• Empty state: a clear "No whiteboard pages yet" with a create action for writers.
• Works in light and dark modes and at the supported breakpoints.
• Tests cover store CRUD and soft delete, permissions for writer, read-only, public and outsider, API and MCP, realtime, changelog, and rendering. This includes checking that no issue-linking UI renders.