Feature/rent to own calculator

[sweetesty]

Closes #953

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This PR builds the rent-to-own plan selector and equity calculator UI as a pure frontend feature with no API calls required.

What was done:

lib/rentToOwnCalc.ts

• Equity accumulation formula encapsulated as a pure utility: calculates monthly rent portion, equity portion, projected ownership date, and total cost over the plan horizon
• Edge case handled: if monthly budget is below the minimum required to reach ownership within the timeline, returns the minimum required payment and flags the plan as infeasible

frontend/app/calculator/rent-to-own/page.tsx

• Dedicated rent-to-own calculator page with correct <title> and <meta> SEO tags
• Prominent placeholder banner making clear rent-to-own is a future product not yet available for booking
• Reachable from the main rent calculator via a tab/link

components/calculator/RentToOwnCalculator.tsx

• Four inputs: property price, deposit percentage, monthly payment capacity, target ownership timeline
• All calculations re-run in real-time as any input changes — no submit button required
• Side-by-side comparison with standard rental (no equity) rendered below the inputs
• Edge case UI: when monthly budget is too low, calculator highlights the minimum required payment instead of showing a broken plan

components/calculator/EquityProgressChart.tsx

• Recharts area/line chart showing equity accumulation over time
• Chart re-renders reactively on every input change
• Empty/loading state handled for initial render before inputs are populated
• Mobile-responsive with readable axis labels at narrow viewports

components/calculator/RentToOwnPlanCard.tsx

• Summary card showing: monthly rent component, equity component, interest, years to ownership, projected ownership date, total cost
• Standard rental comparison card shown side-by-side
• Infeasible plan state renders minimum required payment with clear explanation

Linking

• Calculator linked from property listing detail pages
• Tab or link added on the main rent calculator page

Acceptance criteria met:

• ✅ User can enter property value, deposit %, monthly budget, and desired ownership horizon
• ✅ Chart updates in real-time as inputs change
• ✅ Plan card shows rent component, equity component, interest, and projected ownership date
• ✅ Side-by-side comparison with standard rental
• ✅ Edge case: monthly budget too low shows minimum required payment
• ✅ Calculator fully usable on mobile
• ✅ Page reachable from main rent calculator via tab or link
• ✅ Placeholder copy makes clear rent-to-own is not yet bookable

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