test: setup Vitest workspace and Playwright with first nutrition tests

Vitest:
- Workspace at root (vitest.workspace.ts) running per-package configs
- packages/shared/vitest.config.ts with v8 coverage and 70% thresholds
- Root scripts: test / test:watch / test:coverage

Nutrition module in @ketopath/shared:
- calculateBmr (Mifflin-St Jeor) + calculateTdee with activity multipliers
- ACTIVITY_MULTIPLIERS constants matching CLAUDE.md domain knowledge
- 14 unit tests covering Michele/Laura PRD personas, all sex variants,
  every activity level, and input validation (100% coverage on src/nutrition)

Playwright in apps/web:
- chromium-only project, webServer auto-boot of pnpm dev
- e2e/home.spec.ts smoke test asserts Italian copy and disclaimer

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/shared/package.json

@@ -10,7 +10,9 @@
   },
   "scripts": {
     "typecheck": "tsc --noEmit",
-    "lint": "eslint src --max-warnings=0"
+    "lint": "eslint src --max-warnings=0",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "devDependencies": {
     "@ketopath/eslint-config": "workspace:*",