feat(cups): add manual printer addition by IP/hostname (#1868)

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.

quickshell/Services/CupsService.qml

@@ -479,6 +479,21 @@ Singleton {
         });
     }
 
+    function testConnection(host, port, protocol, callback) {
+        if (!cupsAvailable)
+            return;
+        const params = {
+            "host": host,
+            "port": port,
+            "protocol": protocol
+        };
+
+        DMSService.sendRequest("cups.testConnection", params, response => {
+            if (callback)
+                callback(response);
+        });
+    }
+
     function createPrinter(name, deviceURI, ppd, options) {
         if (!cupsAvailable)
             return;