feat: add performance dashboard skeleton

[Luchistack]

#closes #495

Description:
This PR introduces the foundational structure for the Soroban Analysis Performance Dashboard, enabling visual tracking of scan metrics and rule execution efficiency within GasGuard.

Changes:

Dashboard Scaffolding: Initialized apps/dashboard/performance/stellar/ with a placeholder page.tsx.

Structural Foundation: Prepared the file architecture for future integration of performance visualization components (metrics cards, execution charts, and trend analysis).

Verification:

Confirmed the new directory structure and file creation.

Verified that the dashboard scaffold builds correctly within the monorepo.

#closes