refactor(repo): consolidate openflare-server to root and move subprojects to internal/apps

- Merge all files inside openflare-server to the repository root directory.
- Relocate agent, relay, and flared subprojects from internal/ to internal/apps/.
- Combine docker-compose files and update build context paths to root.
- Update GitHub workflows and Dockerfiles to refer to new directories and package names.
- Rewrite Go package imports across all files.
- Resolve database renew test race condition and clean up docs.

frontend/app/(main)/components/placeholder-page.tsx

@@ -0,0 +1,26 @@
+import type {LucideIcon} from 'lucide-react';
+
+import {Card, CardDescription, CardHeader, CardTitle} from '@/components/ui/card';
+
+interface PlaceholderPageProps {
+  title: string;
+  icon: LucideIcon;
+}
+
+export function PlaceholderPage({title, icon: Icon}: PlaceholderPageProps) {
+  return (
+    <div className="py-6 px-1 space-y-6">
+      <div className="flex items-center gap-2">
+        <Icon className="size-5 text-primary" />
+        <h1 className="text-2xl font-semibold tracking-tight">{title}</h1>
+      </div>
+
+      <Card>
+        <CardHeader>
+          <CardTitle className="text-base">开发中</CardTitle>
+          <CardDescription>该功能正在迁移开发中，敬请期待。</CardDescription>
+        </CardHeader>
+      </Card>
+    </div>
+  );
+}