I built a free UK personal finance app as a single HTML file. Here's why, and how.

A non-developer British backpacker built Vault, a free UK-focused personal finance app as a single HTML file, using Claude Code to write all the code through natural language prompts. The app runs entirely in the browser with localStorage, requiring no accounts, servers, or subscriptions, making user data structurally inaccessible to the creator. It includes UK-specific features like HMRC-compliant CGT tracking, Scottish tax bands, stamp duty calculations, FIRE planning, and investment portfolio tracking.