Make the dashboard a grid of projects with their inbox

DashboardView drops the placeholder for a full-width grid of project cards.
Each card links to the project and lists its inbox cards (status_id === null)
under a "New" heading, or "Nothing new." when empty. Cards are fetched per
project (one GET /projects/{id}/cards each) after the project list resolves.

projects store: fetchProjects() now shares one in-flight request between
concurrent callers (the sidebar and the dashboard mount together).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -18,7 +18,7 @@ Each user owns **projects**, and each project holds ordered **cards**.
 | 8 | Passwordless login — magic-link by default, password login behind a toggle | ✅ done |
 | 9 | Per-project card statuses ("To do" / "Doing" / "Done"); status chip, new cards start with none | ✅ done |
 | 10 | Project view — full-width, tabbed: alphabetical "All tasks" list + "Kanban" board, per-column drag ordering | ✅ done |
-| 11 | Persistent left sidebar — Dashboard link + project list/new-project form; `/` → dashboard placeholder | ✅ done |
+| 11 | Persistent left sidebar (Dashboard link + project list/new-project form); dashboard = grid of projects with their "New" inbox cards | ✅ done |
 
 Registration signs the user in immediately and emails a magic link that verifies
 the address; `user.email_verified` stays `false` until the link is opened. See