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/translations/en.json

@@ -659,6 +659,12 @@
     "reference": "Modules/Settings/DesktopWidgetsTab.qml:84",
     "comment": ""
   },
+  {
+    "term": "Add by Address",
+    "context": "Toggle button to manually add a printer by IP or hostname",
+    "reference": "Modules/Settings/PrinterTab.qml:351",
+    "comment": ""
+  },
   {
     "term": "Adjust the number of columns in grid view mode.",
     "context": "Adjust the number of columns in grid view mode.",
@@ -2429,6 +2435,12 @@
     "reference": "Modules/ControlCenter/Details/BluetoothDetail.qml:297",
     "comment": ""
   },
+  {
+    "term": "Connection failed",
+    "context": "Status message when test connection to printer fails",
+    "reference": "Modules/Settings/PrinterTab.qml:603",
+    "comment": ""
+  },
   {
     "term": "Contains",
     "context": "notification rule match type option",
@@ -3293,6 +3305,12 @@
     "reference": "Services/DMSNetworkService.qml:480",
     "comment": ""
   },
+  {
+    "term": "Discover Devices",
+    "context": "Toggle button to scan for printers via mDNS/Avahi",
+    "reference": "Modules/Settings/PrinterTab.qml:313",
+    "comment": ""
+  },
   {
     "term": "Disk",
     "context": "Disk",
@@ -5267,6 +5285,12 @@
     "reference": "Modals/FileBrowser/FileBrowserContent.qml:241",
     "comment": ""
   },
+  {
+    "term": "Host",
+    "context": "Label for printer IP address or hostname input field",
+    "reference": "Modules/Settings/PrinterTab.qml:462",
+    "comment": ""
+  },
   {
     "term": "Hostname",
     "context": "system info label",
@@ -5345,6 +5369,12 @@
     "reference": "Modules/Settings/NetworkTab.qml:943",
     "comment": ""
   },
+  {
+    "term": "IP address or hostname",
+    "context": "Placeholder text for manual printer address input",
+    "reference": "Modules/Settings/PrinterTab.qml:472",
+    "comment": ""
+  },
   {
     "term": "ISO Date",
     "context": "date format option",
@@ -8261,6 +8291,12 @@
     "reference": "Modals/Greeter/GreeterCompletePage.qml:398",
     "comment": ""
   },
+  {
+    "term": "Port",
+    "context": "Label for printer port number input field",
+    "reference": "Modules/Settings/PrinterTab.qml:486",
+    "comment": ""
+  },
   {
     "term": "Portal",
     "context": "wallpaper transition option",
@@ -8483,6 +8519,12 @@
     "reference": "Modules/Settings/PrinterTab.qml:433",
     "comment": ""
   },
+  {
+    "term": "Printer reachable",
+    "context": "Status message when test connection to printer succeeds",
+    "reference": "Modules/Settings/PrinterTab.qml:603",
+    "comment": ""
+  },
   {
     "term": "Printers",
     "context": "Printers",
@@ -10775,6 +10817,12 @@
     "reference": "Modules/Settings/ThemeColorsTab.qml:1944",
     "comment": ""
   },
+  {
+    "term": "Test Connection",
+    "context": "Button to test connection to a printer by IP address",
+    "reference": "Modules/Settings/PrinterTab.qml:541",
+    "comment": ""
+  },
   {
     "term": "Test Page",
     "context": "Test Page",
@@ -10787,6 +10835,12 @@
     "reference": "Services/CupsService.qml:627",
     "comment": ""
   },
+  {
+    "term": "Testing...",
+    "context": "Button state while testing printer connection",
+    "reference": "Modules/Settings/PrinterTab.qml:541",
+    "comment": ""
+  },
   {
     "term": "Text",
     "context": "shadow color option | text color",