test: close coverage gaps and add frontend test infrastructure

Backend (516 tests, 94% coverage):
- Add azure_openai endpoint/deployment validation tests (config.py -> 100%)
- Add _total_conversations and _avg_turns direct tests (queries.py -> 100%)
- Add transformer edge cases: list content, string checkpoint, invalid JSON,
  malformed message graceful skip (transformer.py -> 93%)
- Add safety combined status_code+error_message interaction tests
- Fix ambiguous 200/422 assertion to strict 422
- Add E2E pagination shape assertions (total, page, per_page, row count)
- Fix ReplayPool mock to respect LIMIT/OFFSET params

Frontend (23 tests, vitest + happy-dom + @testing-library/react):
- Add vitest infrastructure with happy-dom environment
- Add api.ts tests: success, HTTP error, success=false, URL encoding
- Add DashboardPage tests: loading, data, error, empty states
- Add ReplayListPage tests: loading, empty, data, error, status badge classes
- Add ReplayPage tests: loading, steps, empty, error states

frontend/vite.config.ts

@@ -1,8 +1,14 @@
+/// <reference types="vitest" />
 import react from "@vitejs/plugin-react";
 import { defineConfig } from "vite";
 
 export default defineConfig({
   plugins: [react()],
+  test: {
+    environment: "happy-dom",
+    globals: true,
+    setupFiles: ["./src/test-setup.ts"],
+  },
   server: {
     port: 5173,
     proxy: {