Add prettier + prettier-plugin-svelte as devDependencies with a
.prettierrc.json matching the existing code style (tabs, single quotes,
semicolons, trailing commas, 120 char width). Add .prettierignore for
build artifacts and lockfile. Wire up `npm run format` (write) and
`npm run format:check` (CI gate) scripts.
Prettier is chosen over Biome because its prettier-plugin-svelte correctly
preserves <script> block indentation per Svelte convention; Biome's
experimental Svelte formatter flattens script-block indentation to column 0,
producing inconsistent output.
ChickenPige0n
