Feat/272 search filter persistence

[JohnOluB]

Summary

Closes #272

Search filters, sort order, and query text reset on navigation because
AdvancedSearchInterface held all state in memory via useAdvancedSearch.
This PR adds URL-based persistence so users never lose their search state.

Changes

src/hooks/useSearchState.ts (new)

A thin composition hook that wraps useAdvancedSearch and adds:

• URL hydration on mount — restores query, filters, and sort from URL
  params and auto-runs the search if any active state is present.
• URL sync on state change — pushes every update to the URL via
  router.replace({ scroll: false }), keeping the address bar in sync
  without polluting the browser history stack.
• External navigation sync — detects browser back/forward and patches
  internal state from the updated searchParams, guarded by a ref to
  prevent update loops.
• getShareableUrl / copyShareableUrl — returns or copies the
  current canonical URL for sharing filtered views.

URL param mapping:

Param	Field
q	query text
sort	sortBy
types	filter.types
topics	filter.topics
difficulty	filter.difficulty
priceMin / priceMax	filter.priceRange
rating	filter.rating
page	page

Default values are omitted from the URL to keep links clean.

src/components/search/AdvancedSearchInterface.tsx (updated)

• Swapped useAdvancedSearch → useSearchState (one-line change).
• hasSearched now initialises from the URL so a page reload after a
  search correctly restores the results view.
• Added a Share button (lucide Share2) that appears when results
  are active and copies the shareable URL with a ✓ COPIED! confirmation.

Acceptance criteria

• Filters persist across navigation and page reloads
• Filtered views are shareable via URL
• State is restored on return (browser back/forward)
• No new console errors
• Lucide icons used throughout (Share2, Check)
• Tailwind CSS with responsive design

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