Add scripts for E2E and unit test coverage integration

- Added `merge-coverage.ts` to combine Jest and Playwright coverage into a unified report with comprehensive statistics and thresholds.
- Included `convert-v8-to-istanbul.ts` for transforming Playwright V8 coverage into Istanbul format, enabling seamless merging.
- Introduced E2E coverage helpers for Playwright tests (`startCoverage`, `stopAndSaveCoverage`, `withCoverage`) to collect and process both V8 and Istanbul coverage.
- Configured `.nycrc.json` with coverage thresholds and reporting formats for improved visibility and enforcement.

frontend/.nycrc.json

@@ -0,0 +1,34 @@
+{
+  "all": true,
+  "include": [
+    "src/**/*.{js,jsx,ts,tsx}"
+  ],
+  "exclude": [
+    "src/**/*.d.ts",
+    "src/**/*.test.{js,jsx,ts,tsx}",
+    "src/**/__tests__/**",
+    "src/**/*.stories.{js,jsx,ts,tsx}",
+    "src/lib/api/generated/**",
+    "src/**/*.old.{js,jsx,ts,tsx}",
+    "src/components/ui/**",
+    "src/app/dev/**",
+    "src/**/index.{js,jsx,ts,tsx}",
+    "src/lib/utils/cn.ts",
+    "src/middleware.ts"
+  ],
+  "reporter": [
+    "text",
+    "text-summary",
+    "html",
+    "json",
+    "lcov"
+  ],
+  "report-dir": "./coverage-combined",
+  "temp-dir": "./.nyc_output",
+  "sourceMap": true,
+  "instrument": true,
+  "branches": 85,
+  "functions": 85,
+  "lines": 90,
+  "statements": 90
+}