Tooling: ESLint flat config, pinned Prettier, npmrc, strict TS

Copy the issues repo shape, plus the checklist items it is missing.

• Flat eslint.config.js: @eslint/js, typescript-eslint, eslint-plugin-svelte, eslint-config-prettier, includeIgnoreFile(.gitignore), no-undef off, projectService true
• .prettierrc (printWidth 200, tabWidth 2, semi, es5 commas, arrowParens avoid) + prettier-plugin-svelte; prettier pinned as an explicit dev dependency
• .npmrc: save-exact=true, engine-strict=true; engines pinned to Node 22
• tsconfig strict; scripts: lint, format, format:check, typecheck (svelte-check), test:unit, test:e2e