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>

frontend/services/dto/project.ts

@@ -13,6 +13,7 @@ export type Project = {
     bookstackShelfId: number | null
     bookstackShelfName: string | null
     archived: boolean
+    taskCount: number
 }
 
 export type ProjectWrite = {