feat(project) : add delete button for empty projects with confirmation modal

Adds taskCount virtual field on Project entity, delete button in ProjectDrawer
(visible only when taskCount === 0), and a reusable ConfirmDeleteProjectModal.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Entity/Task.php

@@ -82,7 +82,7 @@ class Task
     #[Groups(['task:read', 'task:write'])]
     private ?TaskGroup $group = null;
 
-    #[ORM\ManyToOne(targetEntity: Project::class)]
+    #[ORM\ManyToOne(targetEntity: Project::class, inversedBy: 'tasks')]
     #[ORM\JoinColumn(nullable: false, onDelete: 'CASCADE')]
     #[Groups(['task:read', 'task:write'])]
     private ?Project $project = null;