fix(admin): sidebar hamburger menu for mobile, remove auto Cantaloupe sync

- Replace broken icon-only sidebar collapse with off-canvas hamburger menu
  that slides in from left (hidden by default on <768px)
- Add sidebar overlay backdrop, auto-close on nav/logout
- Rename 'Cantaloupe Sync' to 'Import' in sidebar and 'Excel Import' in page
- Remove 'Sync Now' button and backend subprocess auto-export entirely
- Keep manual Excel file upload as the only import path
- Update empty-state text and upload handler to work without Sync Now btn

cantaloupe_sync.py

@@ -386,7 +386,7 @@ async def run_sync(
     file_path: Optional[str] = Query(None, description="Server-side path to Excel file"),
 ):
     """
-    Run a full Cantaloupe sync from an Excel file.
+    Import an Excel file into a staged batch for review.
 
     Accepts either:
       - An uploaded Excel file (multipart form)
@@ -416,42 +416,10 @@ async def run_sync(
         excel_path = str(p)
         display_path = file_path
     else:
-        # Try subprocess call to cantaloupe export
-        try:
-            import subprocess
-            import sys
-            cantaloupe_dir = os.path.expanduser("~/projects/cantaloupe-downloader")
-            result = subprocess.run(
-                [sys.executable, "-m", "cantaloupe", "export", "--scheduled"],
-                capture_output=True, text=True, timeout=120,
-                cwd=cantaloupe_dir,
-            )
-            if result.returncode != 0:
-                raise HTTPException(
-                    status_code=400,
-                    detail=f"Cantaloupe export failed: {result.stderr[:500]}"
-                )
-            # Parse output to find the exported file path
-            for line in result.stdout.splitlines() + result.stderr.splitlines():
-                line = line.strip()
-                if line and Path(line).exists() and line.endswith((".xlsx", ".xls")):
-                    excel_path = line
-                    display_path = line
-                    break
-            else:
-                raise HTTPException(
-                    status_code=400,
-                    detail="Cantaloupe export ran but produced no recognizable file path. Provide file upload or file_path parameter."
-                )
-        except FileNotFoundError:
-            raise HTTPException(
-                status_code=400,
-                detail="No file provided and cantaloupe module not found. Upload an Excel file or provide file_path."
-            )
-        except HTTPException:
-            raise
-        except Exception as e:
-            raise HTTPException(status_code=500, detail=f"Cantaloupe export error: {e}")
+        raise HTTPException(
+            status_code=400,
+            detail="No file provided. Upload an Excel file to import asset data."
+        )
 
     conn = _get_db()
     try: