Fix username display issues (#6471)

# Description of Changes
Main fixes:
- Fix the display of the username in the bottom left
- Now displays as "User" when not logged in on self-hosted (desktop) and
"Guest" on SaaS when logged in anonymously
- Now updates properly when the user logs in/out in SaaS, desktop and
self-hosted
- Fix incremental build issues in the desktop app that have been here
since the start (I hope at least - I think the issue is that the JLink
is built read-only and then on subsequent builds you get OS errors when
trying to override the JLink with the new version. There's no real need
for it to be read-only that I know of, so we might as well just make it
R/W and ship like that)

frontend/editor/src/core/auth/UseSession.tsx

@@ -1,6 +1,19 @@
 export interface AuthContextType {
   session: null;
   user: { id?: string; email?: string; [key: string]: unknown } | null;
+  /**
+   * Human-readable name to show in the UI for the current session.
+   * - A real identity (username/email/full_name) when the user is signed in.
+   * - A layer-specific placeholder (e.g. "Guest" in SaaS, "User" in
+   *   proprietary) for anonymous sessions.
+   * - null only when there is no user object at all (signed-out, or core
+   *   OSS with no auth context) - consumers can fall back to whatever
+   *   makes sense in their build.
+   *
+   * Each layer derives this from its own native user shape - consumers
+   * should treat the resulting string as opaque display text.
+   */
+  displayName: string | null;
   loading: boolean;
   error: Error | null;
   signOut: () => Promise<void>;
@@ -15,6 +28,7 @@ export function useAuth(): AuthContextType {
   return {
     session: null,
     user: null,
+    displayName: null,
     loading: false,
     error: null,
     signOut: async () => {},