Adds the "Challenges" page (#/utfordringer) with two spin-the-wheel components.

What's included

• Place wheel filled with all venues, with a type filter (All, Nightclub, Pub, Bar, Wine bar)
• Challenge wheel with mock challenges, styled identically
• Wheels styled as ship helms. Segments sit between the spokes, so the pointer always lands on a segment and never on a spoke
• Result dialog with a close button. The result is kept in a field below the wheel and can reopen the dialog
• Compass link next to the place result, opening the venue's website
• Mode toggle to show both wheels, only places or only challenges
• "Spin both" button. When both wheels are spun together, the results are shown in one combined dialog ("Hvor" / "Hva")
• Semantic markup, native <dialog>, keyboard-accessible wheel hub, live region for the result and reduced-motion support

Supporting changes

• New useFetch hook and fetchAllPlaces / fetchChallenges API functions (mock data behind async functions, same pattern as the rest of the app)
• Added the Nattklubb venue type; two mock venues were retyped to it (Nattergal Bar is now Nattergal Klubb)
• Added the #/utfordringer route and linked it from the main navigation

Notes

• Stacked on feat/user-page, so this PR targets that branch until the earlier PRs are merged
• Mock data only, no real API calls yet

Related to #16