feat: add category filter chips to search page

[muffti123]

Summary

Adds a horizontal scrollable row of category filter chips to the Search page, allowing users to narrow raffle results by category without a full page reload.

Changes

client/src/services/raffleService.ts

• searchRaffles now accepts an optional categories?: string[] parameter
• When categories are provided, fires parallel GET /search?q=...&category=... requests (one per selected category) and deduplicates results by raffle ID via a Set<number>

client/src/hooks/useSearch.ts

• Accepts a second categories: string[] argument (defaults to [])
• Uses categories.sort().join(',') as a stable dependency key so the search re-fires when categories change
• Passes categories through to searchRaffles

client/src/pages/Search.tsx

• Reads a comma-separated ?category= from the URL and parses it into selectedCategories via useMemo
• Renders a horizontally scrollable row of rounded filter chips above the results grid
• Active chips are filled pink (#FE3796), inactive chips use an outlined border style
• toggleCategory toggles a category in/out and updates the URL via setSearchParams with replace: true
• useSearch receives both query and selectedCategories

Acceptance Criteria

• Clicking a category chip narrows search results to that category
• Multi-select: multiple category chips can be active simultaneously
• Active categories are reflected in the URL query string (?category=gaming,art)
• Category filters survive page refresh

Categories

Gaming, Electronics, Art, Music, Sports, Collectibles, Other

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Closes #832

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