Allow login to SaaS for desktop instead of offline mode (#4941)

# Description of Changes
Makes the desktop options to sign in with your Stirling account, or sign
into self-hosted:

<img width="608" height="456" alt="image"
src="https://github.com/user-attachments/assets/a49988ab-db3f-4333-b242-790aee5c07c6"
/>

The first option still runs everything locally, just enforces that
you've signed in for now. Future work will enable sending operations
that can't be run locally to the server.

frontend/public/locales/en-GB/translation.json

@@ -5623,15 +5623,23 @@
       "description": "Enter credentials"
     },
     "mode": {
-      "offline": {
-        "title": "Use Offline",
-        "description": "Run locally without an internet connection"
+      "saas": {
+        "title": "Stirling Cloud",
+        "description": "Sign in with your Stirling account"
       },
-      "server": {
-        "title": "Connect to Server",
-        "description": "Connect to a remote Stirling PDF server"
+      "selfhosted": {
+        "title": "Self-Hosted Server",
+        "description": "Connect to your own Stirling PDF server"
       }
     },
+    "saas": {
+      "title": "Sign in to Stirling",
+      "subtitle": "Sign in with your Stirling account"
+    },
+    "selfhosted": {
+      "title": "Sign in to Server",
+      "subtitle": "Enter your server credentials"
+    },
     "server": {
       "title": "Connect to Server",
       "subtitle": "Enter your self-hosted server URL",
@@ -5673,16 +5681,12 @@
     "connection": {
       "title": "Connection Mode",
       "mode": {
-        "offline": "Offline",
-        "server": "Server"
+        "saas": "Stirling Cloud",
+        "selfhosted": "Self-Hosted"
       },
       "server": "Server",
       "user": "Logged in as",
-      "switchToServer": "Connect to Server",
-      "switchToOffline": "Switch to Offline",
-      "logout": "Logout",
-      "selectServer": "Select Server",
-      "login": "Login"
+      "logout": "Log Out"
     },
     "general": {
       "title": "General",