Add dark mode support to soroban-client with persistent theme toggle and system preference fallback

[EtimbukUdofia]

Summary

This PR implements dark mode support for the soroban-client frontend and resolves issue #88.

What is included

• Added a global theme controller with:
  • light and dark theme support
  • system preference fallback by default
  • theme persistence in localStorage
• Added a theme toggle in the header for both desktop and mobile navigation
• Added early theme initialization in the localized app layout to prevent theme flicker during hydration
• Updated global styling tokens and dark mode variant setup
• Updated shared UI components and page wrappers for dark mode compatibility using Tailwind dark utilities
• Added a focused header test to verify:
  • theme toggle behavior
  • localStorage persistence
  • root dark class updates

Issue

Closes #88

Scope Notes

This PR is intentionally scoped to dark mode support and related UI compatibility updates.
Unrelated pre-existing lint/build issues in other areas of soroban-client were not addressed as part of this issue fix.

Validation

• Header theme test suite passes:
  • 1 test suite passed
  • 4 tests passed
• File-level diagnostics on touched dark mode files report no errors

User Impact

• Users can switch between light and dark mode from the header
• Theme preference persists across sessions
• If no preference is saved, the app follows the operating system color scheme
• UI surfaces across updated pages and components now render correctly in both themes

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@EtimbukUdofia Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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