Add a new "Add by Address" flow in the printer settings that allows
users to manually add printers by IP address or hostname, enabling
printing to devices not visible via mDNS/Avahi discovery (e.g.,
printers behind Tailscale subnet routers, VPNs, or across network
boundaries).
Go backend:
- New cups.testConnection IPC method that probes remote printers via
IPP Get-Printer-Attributes with /ipp/print then / fallback
- Input validation with host sanitization and protocol allowlist
- Auth-aware probing (HTTP 401/403 reported as reachable)
- lpadmin CLI fallback for CreatePrinter/DeletePrinter when
cups-pk-helper polkit authorization fails
QML frontend:
- "Add by Address" toggle alongside existing device discovery
- Manual entry form with host, port, protocol fields
- Test Connection button with loading state and result display
- Smart PPD auto-selection by probed makeModel with driverless fallback
- All strings use I18n.tr() with translator context
Includes 20+ unit tests covering validation, probe delegation, TLS
flag propagation, auth error detection, and handler routing.
