chore: add CODEOWNERS and PR template

.github/CODEOWNERS

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+# CODEOWNERS
+# Require review from the repo owner on key paths.
+# Docs: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners
+
+# Default: everything
+* @vincenthouyi
+
+# App code
+/src/ @vincenthouyi
+
+# Tooling / config
+/package.json @vincenthouyi
+/package-lock.json @vincenthouyi
+/vite.config.ts @vincenthouyi
+/tsconfig*.json @vincenthouyi
+/eslint.config.js @vincenthouyi
+/playwright.config.ts @vincenthouyi
+
+# CI / GitHub
+/.github/ @vincenthouyi

.github/pull_request_template.md

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+## What / Why
+
+- **What changed?**
+- **Why?**
+
+## How to test
+
+- [ ] `npm ci`
+- [ ] `npm run lint`
+- [ ] `npm run test`
+- [ ] `npm run test:e2e` (if applicable)
+- [ ] `npm run build`
+
+## Screenshots / video (UI changes)
+
+<!-- Paste screenshots or a short clip. -->
+
+## Risk / Rollback
+
+- **Risk:** (low/med/high) + why
+- **Rollback plan:** (revert PR / toggle flag / etc.)
+
+## Checklist
+
+- [ ] Small PR (focused scope)
+- [ ] Added/updated tests (or explained why not)
+- [ ] No new lint/type errors
+- [ ] Updated docs/README if behavior changed