Add utility bills proof of payment file upload functionality

Changes:
- Updated BillingLocation interface:
  - Added utilBillsProofOfPaymentAttachment field (BillAttachment type)
- Added server action uploadUtilBillsProofOfPayment:
  - Validates PDF file type
  - Serializes file attachment to base64
  - Stores attachment in BillingLocation document
  - Returns success/error status
- Updated ViewLocationCard component:
  - Added file upload input with PDF-only accept
  - Implemented handleFileChange with immediate upload
  - Added upload state management (isUploading, uploadError, attachment)
  - Shows spinner while uploading
  - Input disabled during upload
  - Conditionally renders file input or download link
  - Link displayed after successful upload
- Created route handler for serving proof of payment PDFs:
  - GET /share/proof-of-payment/[id]/route.tsx
  - Fetches attachment from database
  - Converts base64 to binary
  - Returns PDF with proper headers
- Added not-found page for proof of payment route
- Updated middleware to include proof-of-payment in public pages
- Added translations:
  - en: "Upload proof of payment (PDF only)"
  - hr: "Priložite potvrdu o uplati:"

File uploads immediately on selection without page reload.
Only PDF files accepted with client and server-side validation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/lib/actions/locationActions.ts

@@ -580,4 +580,77 @@ export const setSeenByTenant = async (locationID: string): Promise<void> => {
             { _id: locationID },
             { $set: { seenByTenant: true } }
         );
+}
+
+/**
+ * Serializes a file attachment to be stored in the database
+ * @param file - The file to serialize
+ * @returns BillAttachment object or null if file is invalid
+ */
+const serializeAttachment = async (file: File | null) => {
+    if (!file) {
+        return null;
+    }
+
+    const {
+        name: fileName,
+        size: fileSize,
+        type: fileType,
+        lastModified: fileLastModified,
+    } = file;
+
+    if(!fileName || fileName === 'undefined' || fileSize === 0) {
+        return null;
+    }
+
+    // Convert file contents to base64 for database storage
+    const fileContents = await file.arrayBuffer();
+    const fileContentsBase64 = Buffer.from(fileContents).toString('base64');
+
+    return {
+        fileName,
+        fileSize,
+        fileType,
+        fileLastModified,
+        fileContentsBase64,
+    };
+}
+
+/**
+ * Uploads utility bills proof of payment attachment for a location
+ * @param locationID - The ID of the location
+ * @param formData - FormData containing the file
+ * @returns Promise with success status
+ */
+export const uploadUtilBillsProofOfPayment = async (locationID: string, formData: FormData): Promise<{ success: boolean; error?: string }> => {
+    noStore();
+
+    try {
+        const file = formData.get('utilBillsProofOfPaymentAttachment') as File;
+
+        // Validate file type
+        if (file && file.type !== 'application/pdf') {
+            return { success: false, error: 'Only PDF files are accepted' };
+        }
+
+        const attachment = await serializeAttachment(file);
+
+        if (!attachment) {
+            return { success: false, error: 'Invalid file' };
+        }
+
+        const dbClient = await getDbClient();
+
+        // Update the location with the attachment
+        await dbClient.collection<BillingLocation>("lokacije")
+            .updateOne(
+                { _id: locationID },
+                { $set: { utilBillsProofOfPaymentAttachment: attachment } }
+            );
+
+        return { success: true };
+    } catch (error: any) {
+        console.error('Error uploading util bills proof of payment:', error);
+        return { success: false, error: error.message || 'Upload failed' };
+    }
 }