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/eslint.config.js

@@ -0,0 +1,47 @@
+import js from "@eslint/js";
+import svelte from "eslint-plugin-svelte";
+import ts from "typescript-eslint";
+import globals from "globals";
+
+export default [
+  {
+    ignores: [
+      ".svelte-kit",
+      "node_modules",
+      "build",
+      "dist",
+      "**/paraglide/**",
+      "**/lib/paraglide/**",
+    ],
+  },
+  js.configs.recommended,
+  ...ts.configs.recommended,
+  ...svelte.configs["flat/recommended"],
+  {
+    languageOptions: {
+      ecmaVersion: "latest",
+      sourceType: "module",
+      globals: {
+        ...globals.browser,
+      },
+    },
+    rules: {
+      "svelte/no-at-html-tags": "off",
+      "svelte/require-each-key": "off",
+      // TODO: Set ignoreGoto to false when https://github.com/sveltejs/eslint-plugin-svelte/issues/1327 is fixed
+      // The rule doesn't detect resolve() when used with string concatenation for query params
+      "svelte/no-navigation-without-resolve": [
+        "error",
+        { ignoreLinks: true, ignoreGoto: true },
+      ],
+    },
+  },
+  {
+    files: ["**/*.svelte"],
+    languageOptions: {
+      parserOptions: {
+        parser: ts.parser,
+      },
+    },
+  },
+];