ci: add minimal frontend tests, backend coverage tooling, and fix mypy

frontend/vitest.config.ts

@@ -0,0 +1,20 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    environment: "jsdom",
+    setupFiles: ["./src/setupTests.ts"],
+    globals: true,
+    coverage: {
+      provider: "v8",
+      reporter: ["text", "lcov"],
+      reportsDirectory: "./coverage",
+      include: ["src/**/*.{ts,tsx}"],
+      exclude: [
+        "**/*.d.ts",
+        "src/**/__generated__/**",
+        "src/**/generated/**",
+      ],
+    },
+  },
+});