feat(ui): refactor staking leaderboard to responsive card layout (#112)

[Alimzy]

Summary

Resolves #112 by refactoring the Staking Leaderboard table into a responsive, mobile-first layout using Tailwind CSS flex and grid utilities.
closes #112

Changes

• Mobile View (< 768px): Transformed table rows into stacked cards with internal uppercase labels (Rank, Address, Staked Amount), eliminating horizontal scrolling down to 320px viewports.
• Desktop View (≥ 768px): Maintained the multi-column data table structure with sticky/visible headers.
• Text Overflow Fix: Applied address truncation (truncate) and flexible width constraints to prevent layout breakage on small mobile devices.

Acceptance Criteria

• No horizontal scrolling on screens as small as 320px.
• Desktop layout (≥ 1024px) remains a dense data table.
• Information hierarchy (Rank → Address → Staked Amount) preserved across all viewports.
• Exact same dataset retained; no columns removed on mobile.