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/app/manifest.ts

@@ -0,0 +1,21 @@
+import type { MetadataRoute } from "next";
+
+export default function manifest(): MetadataRoute.Manifest {
+  return {
+    name: "Agentic Coding Boilerplate",
+    short_name: "Agentic",
+    description:
+      "Complete agentic coding boilerplate with authentication, database, AI integration, and modern tooling",
+    start_url: "/",
+    display: "standalone",
+    background_color: "#ffffff",
+    theme_color: "#000000",
+    icons: [
+      {
+        src: "/favicon.ico",
+        sizes: "any",
+        type: "image/x-icon",
+      },
+    ],
+  };
+}