feat: add receipt upload functionality for Einnahmen and Beleg routes

- Implemented upload and retrieval of receipts (Belege) associated with Einnahmen entries.
- Added new API routes for uploading and deleting receipts.
- Updated the Einnahmen model to include a `belegUrl` field for storing receipt references.
- Enhanced the Einnahmen page to support file uploads and display existing receipts.
- Introduced drag-and-drop functionality for file uploads and improved user feedback during uploads.
- Added necessary validation for file types and sizes during uploads.

app/routes/api.einnahmen.ts

@@ -10,6 +10,7 @@ const createSchema = z.object({
   zahlungsart: z.enum(["KASSE", "BANK"]).default("BANK"),
   datum: z.string().min(1),
   beschreibung: z.string().optional(),
+  belegUrl: z.string().optional(),
 });
 
 /**
@@ -105,6 +106,7 @@ export async function action({ request }: { request: Request }) {
       steuersatz: parsed.data.steuersatz,
       zahlungsart: parsed.data.zahlungsart,
       isBusinessRecord: true,
+      belegUrl: parsed.data.belegUrl || null,
     },
   });