Rename "SIGN IN" / "SIGN UP" to "LOGIN" / "REGISTER" on login page

### Your use case

#### What would you like to do?
Change the two primary action labels on the app’s login page:
- Replace the current "SIGN IN" label with "LOGIN"
- Replace the current "SIGN UP" label with "REGISTER"

Additionally, modify the account-creation error message to include a one-click alternative back to the login flow, for example: "Did you mean to log in instead? Click here to log in." Ensure this link opens the login screen directly.

#### Why would you like to do it?
- I do not accept user-initiated registrations on my homeserver; I create accounts manually. Users repeatedly click "SIGN UP" by mistake trying to login when they receive the credentials and receive the error "Your homeserver needs to upgrade to support Matrix Authentication Service and account creation", which they do not understand.
- This generates dozens of support requests per month from both non-native and native English speakers.
- Using "LOGIN" and "REGISTER" aligns with common web conventions and will reduce confusion and support load.

#### How would you like to achieve it?
Update the UI strings for the login page so labels read "LOGIN" and "REGISTER"

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

### Are you willing to provide a PR?

No