feat: EMI calculator that compares all banks at once

[ayushcmd]

Most EMI calculators just show one bank. Wanted to build one
where you enter your loan amount and tenure and it shows
what your EMI would be at each bank — so you can actually
pick the cheapest option.

Also want to show total interest paid over the full tenure
because that number is usually shocking and more useful
than the monthly EMI.

To do:

• Basic EMI formula implementation
• Use rates from the rates.csv (Issue feat: FD and loan rate comparison across all 10 banks #2)
• Sort results cheapest to most expensive
• Add "you save ₹X vs most expensive option" line