build(frontend): make eslint config usable

Ignore build/dist output, enable the jsx-runtime rule set (React import
no longer required), and treat _-prefixed identifiers as intentionally
unused. Error count drops from ~59k to ~1.2k remaining upstream
violations for a later cleanup pass.

Authored By: TDvorak <info@tdvorak.dev>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

frontend/eslint.config.js

@@ -4,6 +4,9 @@ import globals from "globals";
 import tseslint from "typescript-eslint";
 
 export default defineConfig([
+  {
+    ignores: ["build/**", "dist/**", "node_modules/**"],
+  },
   {
     files: ["**/*.{js,mjs,cjs,ts,mts,cts,jsx,tsx}"],
     languageOptions: { globals: globals.browser },
@@ -15,4 +18,17 @@ export default defineConfig([
   },
   tseslint.configs.recommended,
   pluginReact.configs.flat.recommended,
+  pluginReact.configs.flat["jsx-runtime"],
+  {
+    rules: {
+      "@typescript-eslint/no-unused-vars": [
+        "error",
+        {
+          argsIgnorePattern: "^_",
+          varsIgnorePattern: "^_",
+          caughtErrorsIgnorePattern: "^_",
+        },
+      ],
+    },
+  },
 ]);