[Challenge] Add Challenge Search Functionality

Problem

Users cannot search for challenges by keyword. Currently the only way to find challenges is browsing or filtering by topic.

Goals

• Add a search box to the challenges page
• Search across title, description, and tags
• Real-time filtering as user types (debounced)
• Maintain compatibility with existing topic filters and pagination

Acceptance Criteria

• Search input on /challenges page
• Filters challenges by title and description (case-insensitive)
• Works with existing topic and tab filters
• Debounced input (300ms) to avoid excessive re-renders
• URL updates with search query for shareability (?q=escrow)
• Clear search button

Alignment

Improves discoverability as challenge count grows.

Metadata

Bounty: $5 USDC Funding Threshold: $5 USDC
Difficulty: Easy