[Feature] Add percentage calculator

[yeleepark]

Summary

Add a practical percentage calculator for common business calculations such as percentage change, discounts, margins, and ratio questions.

Why

Percentage calculations come up often in daily work across operations, sales, marketing, finance, and product analysis. This would add a broadly useful non-developer utility to oddkit while keeping the tool small, fast, and local-first.

Proposed scope

• Calculate percentage increase and decrease
• Calculate discounts and final prices
• Calculate margin and markup
• Answer ratio questions such as "A is what percent of B?"
• Support quick copy for calculated results
• Handle invalid or missing inputs with clear error states
• Process everything locally in the browser

UX notes

• Use separate compact calculator modes instead of one overloaded form
• Keep formulas or labels visible enough to avoid ambiguity
• Suggested microcopy: // small math, fewer tabs

Acceptance criteria

• Common percentage calculations return accurate results
• Empty, invalid, and divide-by-zero cases are handled without crashing
• Results can be copied to the clipboard
• No input data is uploaded to a server