Add Cantaloupe Sync admin UI page

- New 'Cantaloupe Sync' nav item in sidebar with sync icon
- Section 1: Sync Overview showing latest batch status, diff badges
- Section 2: Pending Batch Review with approve/reject buttons and diff detail
- Section 3: Import History with expandable rows for full diff view
- Added static file serving (FileResponse) to admin_server.py
- Fixed formatDate to handle space-separated SQLite datetimes
- Dark theme matching existing admin UI, vanilla JS with fetch()

admin_server.py

@@ -24,6 +24,7 @@ from typing import Optional
 from fastapi import FastAPI, HTTPException, Query, Request, UploadFile, File
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse, StreamingResponse
+from fastapi.staticfiles import StaticFiles
 from pydantic import BaseModel
 
 from cantaloupe_sync import router as cantaloupe_router, create_sync_tables
@@ -354,6 +355,20 @@ app.add_middleware(
 # Mount Cantaloupe sync routes
 app.include_router(cantaloupe_router)
 
+# Serve admin frontend SPA (single index.html, no external assets needed)
+from fastapi.responses import FileResponse
+
+FRONTEND_INDEX = Path(__file__).parent / "static" / "index.html"
+
+
+@app.get("/")
+@app.get("/{full_path:path}")
+async def serve_frontend(full_path: str = ""):
+    """Serve the admin SPA — all routes return index.html."""
+    if FRONTEND_INDEX.is_file():
+        return FileResponse(str(FRONTEND_INDEX))
+    return JSONResponse({"detail": "Frontend not found"}, status_code=404)
+
 
 # ─── Auth Middleware ────────────────────────────────────────────────────────