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/proprietary/routes/Login.test.tsx

@@ -109,6 +109,7 @@ describe("Login", () => {
     vi.mocked(useAuth).mockReturnValue({
       session: null,
       user: null,
+      displayName: null,
       loading: false,
       error: null,
       signOut: vi.fn(),
@@ -155,6 +156,7 @@ describe("Login", () => {
     vi.mocked(useAuth).mockReturnValue({
       session: mockSession,
       user: mockSession.user,
+      displayName: mockSession.user.username,
       loading: false,
       error: null,
       signOut: vi.fn(),
@@ -178,6 +180,7 @@ describe("Login", () => {
     vi.mocked(useAuth).mockReturnValue({
       session: null,
       user: null,
+      displayName: null,
       loading: true,
       error: null,
       signOut: vi.fn(),