Add firmware update helper for LAN-only printers

Enables checking and uploading firmware updates for printers operating
  in LAN-only mode without Bambu Cloud connectivity.

  Features:
  - Automatic firmware version checking against Bambu Lab servers
  - Orange "Update" badge on printer cards when updates available
  - Firmware update modal with version info and release notes
  - One-click firmware upload to printer SD card via FTP
  - Real-time upload progress (actual bytes transferred)
  - Step-by-step instructions for triggering update from printer
  - Local firmware caching for faster re-uploads
  - Supports all Bambu Lab printer models

  New files:
  - backend/app/services/firmware_check.py - Version checking service
  - backend/app/services/firmware_update.py - Upload orchestration
  - backend/app/api/routes/firmware.py - REST API endpoints

  Also includes:
  - FTP upload progress callback support
  - 10-minute upload timeout protection
  - Firmware cache directory in .gitignore

.gitignore

@@ -32,6 +32,9 @@ npm-debug.log*
 # Archive files (user data)
 archive/
 
+# Firmware cache (downloaded firmware files)
+firmware/
+
 # Virtual printer (auto-generated certs and uploads)
 virtual_printer/