chore: add ESLint + Prettier, fix all lint errors
- Install eslint, typescript-eslint, eslint-plugin-astro, prettier,
prettier-plugin-astro, globals
- Add flat eslint.config.mjs with recommended rules for JS, TS, Astro
- Add .prettierrc with astro plugin
- Add lint, lint:fix, format, format:check scripts
- Fix prefer-const, no-unused-vars, no-useless-assignment across codebase
- Fix AdBanner.astro JSX fragment for Prettier compatibility
- Add eslint-disable around Yandex/Top.Mail.Ru inline scripts
- Auto-format entire codebase with Prettier
emil
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