feat: link dashboard stat cards to pre-filtered contract views

[Chrisland58]

Allow users to jump from dashboard summary cards into pre-filtered contract views

The three stat cards on the dashboard (Contracts Watched, Alerts Today, Active Webhooks) were
static display-only elements. This PR makes them clickable links that navigate to the contracts
list with an appropriate filter applied.

Changes:

• app/dashboard/page.tsx — converted stat cards from
  to with hover:border-zinc-600
  hover state, pointing to /contracts, /contracts?filter=alerts, and /contracts?filter=webhooks
• app/contracts/page.tsx — reads ?filter via useSearchParams() and filters the contract list
  accordingly:
  • filter=alerts — contracts with at least one alert since midnight today
  • filter=webhooks — contracts with an active webhook URL
  • no param — all contracts (unchanged behaviour)
  • Active filter is shown in the subtitle with a "clear filter" link back to /contracts

Testing:

• Navigate to the dashboard with contracts registered; click each stat card and verify the
  correct subset is shown
• Verify "clear filter" returns to the full list
• Verify no regression when no contracts are registered
• closes Add dashboard quick filters #77

[drips-wave]

@Chrisland58 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! 🚀

Learn more about application limits