feat: add BBR-based fee calculator to Opkrævningsoverblik

[yepzdk]

Summary

• New "Beregner" tab in the Opkrævningsoverblik prototype calculating four BBR-driven municipal fees: rottebekæmpelse (with >250 m² rate split), renovation, skorstensfejer, and ejendomsskat (grundskyld)
• Editable mock-BBR per test user with per-field reset, live recalculation, and "hvad-hvis" scenario chips (add carport / brændeovn / expand bolig / reset)
• "Se beregning →" link from each charge detail switches to the calculator and highlights the relevant card; new skorstensfejer charge type added
• Mock-up banner ("Dette er en mock-up, ikke det rigtige eller endelige produkt") at the top of the prototype, matching the pattern used in other research mocks

Test plan

• npm run docs:dev → open /research-projects/projects/opkraevningsoverblik/mocks/index.html
• Switch between Anders / Maria / Lars and verify the four fee cards show different totals; Lars's rottebekæmpelse should hit the >250 m² rate split (≈ 297,50 kr)
• Edit grundareal / grundværdi / bygnings-felter and confirm cards update live and the "ift. BBR" delta appears
• Try each "Hvad hvis"-chip and confirm the BBR panel updates
• Click the per-field ↺ reset and "Nulstil alt" — values return to BBR defaults
• From the Oversigt tab, open a rottebekæmpelse / renovation / skorstensfejer / ejendomsskat charge → click "Se beregning →" → lands on the right card with a brief highlight
• Verify the mock-up banner is visible at the top of the page on all tabs
• Check mobile width (~375px) layout still works
• npm run docs:build runs cleanly

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