feat: add search bar to bounties page (Bounty #823)

[zp6]

Bounty Search Bar

Closes #823

Implementation

• Search input with 300ms debounced filtering
• Clear button to reset search
• Keyboard shortcut: / to focus, Escape to clear
• Works alongside existing status and language filters

Usage

<BountySearch onSearch={(query) => filterBounties(query)} />

Acceptance Criteria

• Search bar visible on /bounties page
• Typing filters bounties in real-time (debounced)
• Works alongside existing filters

Wallet: zp6