Add cancel confirmation and cancellable statuses

Introduce a unified CANCELLABLE_STATUSES constant and add confirmation UI for cancelling requests. RequestActionsDropdown and RequestCard now show a ConfirmModal before cancelling and use the shared CANCELLABLE_STATUSES to gate cancel actions. The API route imports the constant to enforce server-side validation and uses Prisma.DbNull for selectedTorrent when withdrawing an awaiting-approval request. Tests updated to expect Prisma.DbNull. Improves UX and centralizes cancel logic to avoid duplicated status lists.

src/lib/constants/request-statuses.ts

@@ -5,3 +5,12 @@
 
 /** Terminal statuses indicating a request has been fulfilled and files are ready */
 export const COMPLETED_STATUSES = ['available', 'downloaded'] as const;
+
+/** Statuses from which a request can be cancelled (server-enforced and UI-gated) */
+export const CANCELLABLE_STATUSES = [
+  'pending',
+  'searching',
+  'downloading',
+  'awaiting_search',
+  'awaiting_approval',
+] as const;