feat: swap 404 page H1 to show username availability as primary heading

[devin-ai-integration]

What does this PR do?

Swaps the copy hierarchy on the 404 page so that the username availability message ("The username X is still available. Register now.") is promoted to the <h1> heading, and "This page does not exist." becomes the smaller subtext below it.

Before:

• H1 (large): "This page does not exist."
• Subtext (text-lg): "The username X is still available. Register now."

After:

• H1 (large): "The username X is still available. Register now."
• Subtext (text-lg): "This page does not exist."

The isSubpage case (including "Booking not found") is left unchanged — only the primary 404 page layout is affected.

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A — no docs changes needed.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

1. Visit any nonexistent user URL on Cal.com (e.g. cal.com/nonexistent-user-abc123)
2. Verify the 404 page shows:
   • "Error 404" (small uppercase label)
   • H1: "The username /nonexistent-user-abc123 is still available. Register now." (large heading, linked)
   • Below H1: "This page does not exist." (smaller text-lg)
3. Visit a subpage URL (e.g. cal.com/some-user/some-event) to confirm that case still shows "This page does not exist." as the H1 with "check spelling" subtext.

Human Review Checklist

• Verify the <a> nested inside the <h1> (IS_CALCOM branch) renders and behaves correctly — link should still be clickable and styled
• Confirm the non-IS_CALCOM branch (self-hosted) also looks correct with the username in the H1
• Confirm the isSubpage / booking-not-found case is visually unaffected

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have checked if my changes generate no new warnings

[devin-ai-integration]

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[cubic-dev-ai]

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