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/src/app/api/diagnostics/route.ts

@@ -1,4 +1,6 @@
+import { headers } from "next/headers";
 import { NextResponse } from "next/server";
+import { auth } from "@/lib/auth";
 
 type StatusLevel = "ok" | "warn" | "error";
 
@@ -32,6 +34,14 @@ interface DiagnosticsResponse {
 }
 
 export async function GET(req: Request) {
+  // Require authentication for diagnostics endpoint
+  const session = await auth.api.getSession({ headers: await headers() });
+  if (!session) {
+    return NextResponse.json(
+      { error: "Unauthorized. Please sign in to access diagnostics." },
+      { status: 401 }
+    );
+  }
   const env = {
     POSTGRES_URL: Boolean(process.env.POSTGRES_URL),
     BETTER_AUTH_SECRET: Boolean(process.env.BETTER_AUTH_SECRET),
@@ -137,7 +147,7 @@ export async function GET(req: Request) {
     database: {
       connected: dbConnected,
       schemaApplied,
-      error: dbError,
+      ...(dbError !== undefined && { error: dbError }),
     },
     auth: {
       configured: authConfigured,