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>

src/components/auth/sign-in-button.tsx

@@ -1,7 +1,7 @@
 "use client";
 
-import { signIn, useSession } from "@/lib/auth-client";
 import { Button } from "@/components/ui/button";
+import { signIn, useSession } from "@/lib/auth-client";
 
 export function SignInButton() {
   const { data: session, isPending } = useSession();

src/components/auth/sign-out-button.tsx

@@ -1,8 +1,8 @@
 "use client";
 
-import { signOut, useSession } from "@/lib/auth-client";
-import { Button } from "@/components/ui/button";
 import { useRouter } from "next/navigation";
+import { Button } from "@/components/ui/button";
+import { signOut, useSession } from "@/lib/auth-client";
 
 export function SignOutButton() {
   const { data: session, isPending } = useSession();

src/components/auth/user-profile.tsx

@@ -1,7 +1,8 @@
 "use client";
 
-import { useSession, signOut } from "@/lib/auth-client";
-import { SignInButton } from "./sign-in-button";
+import Link from "next/link";
+import { useRouter } from "next/navigation";
+import { User, LogOut } from "lucide-react";
 import { Avatar, AvatarImage, AvatarFallback } from "@/components/ui/avatar";
 import {
   DropdownMenu,
@@ -11,9 +12,8 @@ import {
   DropdownMenuSeparator,
   DropdownMenuTrigger,
 } from "@/components/ui/dropdown-menu";
-import Link from "next/link";
-import { useRouter } from "next/navigation";
-import { User, LogOut } from "lucide-react";
+import { useSession, signOut } from "@/lib/auth-client";
+import { SignInButton } from "./sign-in-button";
 
 export function UserProfile() {
   const { data: session, isPending } = useSession();