Fix nested main tags and missing Header in soroban-client page.tsx

[Majormaxx]

Description

This PR resolves Issue #36 by addressing invalid HTML semantics and missing components in the soroban-client application.

Improvements:

1. Fixed Nested <main> tags: Replaced the outer <main> element with a <div> to adhere to valid HTML semantics.
2. Rendered Missing Header: The Header component was imported but not rendered. It is now correctly placed in the component tree.
3. Layout Integrity: Added flex flex-col to the outer wrapper to ensure the Header and Main content are properly spaced and accessible.

Verification Proof

• Build Integrity: Ran npm run build in soroban-client. Result: ✓ Compiled successfully.
• UI Verification: Confirmed in the browser that the Header is rendering correctly.
• DOM Verification: Verified via JavaScript that exactly one <main> tag exists in the DOM.

Closes #36