Fix hydration mismatch in welcome page

[egoing]

• Introduce appUrl state in Content component initialized to server-side value 'https://otu.ai'.
• Update appUrl to window.location.origin in useEffect to handle client-side rendering correctly.
• Replace getAppUrl() with appUrl in <a> tag to ensure initial hydration matches server HTML.
• This resolves the "Text content does not match server-rendered HTML" error caused by differing href attributes.

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PR created automatically by Jules for task 12743463071525629959 started by @egoing

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