feat: comprehensive boilerplate improvements

Security & Stability:
- Add Next.js 16 proxy.ts for BetterAuth cookie-based auth protection
- Add rate limiting for API routes (src/lib/rate-limit.ts)
- Add Zod validation for chat API request bodies
- Add session auth check to chat and diagnostics endpoints
- Add security headers in next.config.ts (CSP, X-Frame-Options, etc.)
- Add file upload validation and sanitization in storage.ts

Core UX Components:
- Add error boundaries (error.tsx, not-found.tsx, chat/error.tsx)
- Add loading states (skeleton.tsx, spinner.tsx, loading.tsx files)
- Add toast notifications with Sonner
- Add form components (input.tsx, textarea.tsx, label.tsx)
- Add database indexes for performance (schema.ts)
- Enhance chat UX: timestamps, copy-to-clipboard, thinking indicator,
  error display, localStorage message persistence

Polish & Accessibility:
- Add Open Graph and Twitter card metadata
- Add JSON-LD structured data for SEO
- Add sitemap.ts, robots.ts, manifest.ts
- Add skip-to-content link and ARIA labels in site-header
- Enable profile page quick action buttons with dialogs
- Update Next.js 15 references to Next.js 16

Developer Experience:
- Add GitHub Actions CI workflow (lint, typecheck, build)
- Add Prettier configuration (.prettierrc, .prettierignore)
- Add .nvmrc pinning Node 20
- Add ESLint rules: import/order, react-hooks/exhaustive-deps
- Add stricter TypeScript settings (exactOptionalPropertyTypes,
  noImplicitOverride)
- Add interactive setup script (scripts/setup.ts)
- Add session utility functions (src/lib/session.ts)

All changes mirrored to create-agentic-app/template/

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

create-agentic-app/template/.claude/agents/polar-payments-expert.md

@@ -5,7 +5,7 @@ model: sonnet
 color: green
 ---
 
-You are an elite Polar payments integration specialist with uncompromising standards for payment security, reliability, and best practices. Your expertise is in implementing Polar (polar.sh) payment solutions in Next.js 15+ applications.
+You are an elite Polar payments integration specialist with uncompromising standards for payment security, reliability, and best practices. Your expertise is in implementing Polar (polar.sh) payment solutions in Next.js 16+ applications.
 
 ## Core Principles
 
@@ -18,7 +18,7 @@ You are an elite Polar payments integration specialist with uncompromising stand
    - Verify that your guidance matches the latest Polar API specifications
    - Cross-reference multiple sources when available
 
-3. **Next.js 15+ Compatibility**: All implementations must be compatible with Next.js 15 App Router patterns, including:
+3. **Next.js 16+ Compatibility**: All implementations must be compatible with Next.js 16 App Router patterns, including:
    - Server Components vs Client Components usage
    - Server Actions for mutations
    - API route handlers for webhooks
@@ -54,7 +54,7 @@ When assigned a task, follow this strict process:
 3. Add detailed comments explaining security-critical sections
 4. Implement proper logging for debugging (without exposing sensitive data)
 5. Use TypeScript with strict typing
-6. Follow Next.js 15+ conventions (Server Actions, route handlers)
+6. Follow Next.js 16+ conventions (Server Actions, route handlers)
 7. Ensure webhook endpoints are properly secured
 8. Implement idempotency keys where required