refactor: update GitHub installation completion message

[mustafajw07]

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• I have read the Contributing Guide.
• I have added information to docs/ if required so people know about the feature.
• I have filled in the "Changes" section below.
• I have filled in the "How did you test this code" section below.

Changes

Contributes to 7143
Updated the GitHub integration setup completion view in GitHubSetupPage to correctly match the required UI acceptance criteria:

• Removed the old, un-styled <h1>Installation Completed</h1> layout.
• Added the emphasised text: "GitHub installation completed!"
• Added the smaller print optional text beneath it: "You may close this window."
• Ensured the setup form inputs are properly hidden during the completion view.

How did you test this code?

Tested manually on the local frontend dev server:

1. Ran the frontend via ENV=local npm run dev.
2. Bypassed the OAuth flow by setting the success state in the browser DevTools console: localStorage.setItem('githubIntegrationSetupFromFlagsmith', 'install');
3. Navigated directly to /github-setup.
4. Visually verified that the empty layout rendered correctly with the new emphasised text and smaller-print sub-text.
5. Ran frontend linting and typechecking successfully.

[claude]

Claude Code Review

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[vercel]

@mustafajw07 is attempting to deploy a commit to the Flagsmith Team on Vercel.

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[emyller]

@mustafajw07 Thanks for your contribution! I'm afraid, however, this patch does not meet the Acceptance Criteria specified in #7143 yet.

If you'd be so kind also, please add a screenshot to the PR description — that makes it easier to review! Note the window size of the screenshot, given this page opens as a small popup containing the GitHub OAuth flow (see issue examples).

[mustafajw07]

containing

Apologies, i will take i look again.

[Zaimwa9]

Removing my approval after reading the threads, my bad