Fix Cantaloupe Sync: use sys.executable + add Excel file upload button

- Changed subprocess from 'python' to 'sys.executable' (fixes FileNotFoundError
  in systemd — venv python not in system PATH)
- Added '📤 Upload Excel' button to admin frontend sync page
- uploadSyncFile() sends file to the same /api/admin/cantaloupe/sync endpoint
  (which already supported file upload — frontend just didn't expose it)
- Users can now use 'Sync Now' for auto-download or 'Upload Excel' for manual

cantaloupe_sync.py

@@ -419,9 +419,10 @@ async def run_sync(
         # Try subprocess call to cantaloupe export
         try:
             import subprocess
+            import sys
             cantaloupe_dir = os.path.expanduser("~/projects/cantaloupe-downloader")
             result = subprocess.run(
-                ["python", "-m", "cantaloupe", "export", "--scheduled"],
+                [sys.executable, "-m", "cantaloupe", "export", "--scheduled"],
                 capture_output=True, text=True, timeout=120,
                 cwd=cantaloupe_dir,
             )