feat: Advanced bounty search with multi-select filters, reward slider, and saved filter sets (Closes #842)

[sepulchralvoid666]

What this does

Implements advanced search and filtering capabilities for the bounty grid, allowing contributors to filter by programming language, tier, domain, reward range, and deadline.

New Component: BountyFilterPanel

Full-featured filter panel with animated show/hide:

Multi-Select Chip Filters

• Languages: TypeScript, Rust, Solidity, Python, Go, JavaScript, C++, Swift, Kotlin, Move
• Tiers: T1, T2, T3
• Domains: Frontend, Backend, Smart Contracts, SDK, Infrastructure, DevOps, Mobile, AI/ML, Security, Documentation
• Each chip toggles selection with visual check mark

Reward Range Slider

• Price presets: All, <100K, 100K-500K, 500K-1M, 1M+ $FNDRY
• Custom min/max range sliders with accent-emerald styling
• Presets and sliders sync bidirectionally

Deadline Range Picker

• Date inputs for deadline after/before filtering
• Useful for time-sensitive bounty hunters

Saved Filter Sets

• Save current filter configuration with custom name
• Persisted to localStorage across sessions
• One-click apply and delete saved sets
• Full filter state restoration including search query and status

Updated: BountyGrid

• Integrated filter panel with framer-motion AnimatePresence for smooth slide animation
• Client-side filtering for all multi-select, reward range, and deadline fields (works regardless of backend support)
• Server-side params for tier, reward_min, reward_max passed to API
• Filter toggle button with active filter count badge
• Context-aware empty states: search miss, filter miss, no bounties
• Clear all filters action

How to test

1. Navigate to bounties page
2. Click Filters button — panel slides down
3. Toggle language/tier/domain chips — grid filters immediately
4. Use reward presets or drag sliders
5. Set deadline ranges
6. Save a filter set, clear filters, then re-apply saved set
7. Verify filter persistence after page reload

Closes #842

Wallet: 2JHa4QQWNV7AGSGsVeX1cwjZtFcmTTomb1TrJmQthoh3