feat: add interactive cost calculator to landing page (#104)

[Henilll]

Closes #104

Adds an interactive JS cost calculator to the landing page's Benchmarks section.

• Ported the exact dynavec_cost() formula and pricing constants from benchmarks/cost_model.py into vanilla JS (matches script.js's existing style — no framework, IIFE-wrapped)
• Inputs: vectors, dimension, queries/month, writes/month — live recalculation on input
• Verified output against python -m benchmarks.cost_model at two scales (1M vectors and 1B vectors) — matches to the cent, aside from expected floating-point rounding
• Styled to match the existing Benchmarks section (same border, radius, spacing, and responsive breakpoints as the cost table above it)