fix(app): widen new-session prompt beyond logo

[iamdavidhill]

Summary

• Widen the new-session prompt from 720px to 880px.
• Keep the OpenCode logo centered at a maximum width of 720px, so the prompt extends 80px beyond either side on wide windows.
• Preserve responsive sizing for smaller windows.

Validation

• bun typecheck in packages/app passed.
• Full pre-push typechecks passed (33 tasks) with Bun 1.4.1.
• git diff --check passed.
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