feat: refresh home page copy and surface full dashboard navigation

[Jokay1997]

Summary

Replaces the stale wallet-related copy on the landing page and surfaces all primary dashboard routes as navigable cards.

Changes

Home Page (src/app/page.tsx)

• Replaced "Connect your Stellar wallet..." paragraph with operator dashboard copy
• Added navigation cards for all primary routes using the shared Card component:
  • /quote, /pairs, /stats (existing links preserved)
  • /admin, /api-keys, /events, /webhooks, /docs (new cards)
• Every link has focus-visible:outline styling for accessibility
• Main content skip target (#main-content + focus:outline-none) preserved

Tests (src/tests/page.test.tsx)

• Extended to assert all 8 route links render with correct href
• Verifies no stale "Connect your Stellar wallet" copy remains
• Checks operator dashboard description renders
• Confirms skip-target accessibility attributes

Closes #65

[mikewheeleer]

thanks for the effort @Jokay1997! 🙏 issue #65 isn't assigned to you, and we merge from the assigned contributor to keep the campaign fair. please claim an open unassigned issue first, then open your PR. closing for now — hope to see it back 🙌