feat: Add search bar with debounced filtering (Closes #823)

[sepulchralvoid666]

What this does

Adds a search bar to the BountyGrid component with debounced input filtering.

Changes

• Search bar UI: Full-width input with Search icon, X clear button, placeholder text, emerald focus ring
• Debounced input: 300ms debounce avoids excessive API calls while typing
• Server-side filtering: Passes search param to listBounties API call
• Client-side fallback: Filters locally on title, description, and skills fields if backend does not support the search param
• Updated BountiesListParams: Added search?: string to the API params interface
• Empty state: Shows contextual message when search returns no results ("No results for X")

How to test

1. Navigate to the bounties page
2. Type in the search bar — results filter after 300ms
3. Click X to clear search
4. Verify empty state message shows search term
5. Verify API calls include search query parameter

Closes #823

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