Add a "new card" form to each kanban column

Each column gets its own form at the bottom (styled like the sidebar
inbox's), creating the card directly in that status, appended after
its existing cards.

- API: POST /projects/{id}/cards takes an optional status_id, which
  must belong to the project (422 otherwise); omitted, it still
  defaults to the project's first status as before. Position is
  already "end of that status" for free -- createInProject() already
  ranks by (owner, project, status).
- cards store: add() takes an optional statusId, forwarded as
  status_id when given.
- ProjectView: one draft string per status (keyed by status id) so
  typing in one column doesn't touch another's, mirroring the
  per-status independence the columns already have for reordering.

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

src/Http/Controllers/CardController.php

@@ -45,8 +45,9 @@ final class CardController extends ProjectScopedController
     /**
      * POST /api/projects/{projectId}/cards
      *
-     * Creates the card directly in the project, in its first status (a
-     * project card always has one -- see the invariant on the `cards` table).
+     * Creates the card directly in the project, at the end of the given
+     * status -- or the project's first status if none is given (a project
+     * card always has one -- see the invariant on the `cards` table).
      */
     public function store(Request $request, Response $response, array $args): Response
     {
@@ -56,12 +57,17 @@ final class CardController extends ProjectScopedController
         $text = $validator->requiredString('text', self::TEXT_MAX);
         $complete = $validator->optionalBool('complete') ?? false;
         $position = $validator->optionalInt('position', 0);
+        $statusId = $validator->optionalInt('status_id', 1);
         $validator->assert();
 
+        if ($statusId !== null && $this->statuses->findInProject($statusId, $projectId) === null) {
+            throw new ApiException('That status does not belong to this project.', 422);
+        }
+
         $card = $this->cards->createInProject(
             $this->user($request)['id'],
             $projectId,
-            $this->firstStatusId($projectId),
+            $statusId ?? $this->firstStatusId($projectId),
             $text,
             $complete,
             $position,