feat(frontend): add ESLint and Prettier with Svelte support

- Install eslint, prettier and related plugins
- Add lint and format npm scripts
- Configure eslint.config.js with Svelte + TypeScript rules
- Configure .prettierrc with Svelte plugin
- Fix code to comply with lint rules:
  - Use resolve() for navigation links
  - Use SvelteMap for reactive maps
  - Use writable  instead of  +
  - Remove unused imports and variables

Note: ignoreGoto is set to true due to eslint-plugin-svelte#1327

frontend/.prettierignore

@@ -0,0 +1,8 @@
+node_modules
+.svelte-kit
+paraglide
+build
+dist
+.env
+.env.*
+!.env.example