docs: vendor the documentation site into the monorepo

Fold the standalone 3x-ui-docs project (Next.js 16 + Fumadocs, deployed to
docs.sanaei.dev) into docs/ so the panel and its documentation share a single
source of truth, the way sing-box keeps its docs in-tree. The old repo becomes
redundant and can be retired.

- Import the full site under docs/ (app, components, content, lib, public,
  scripts, config). The self-contained pnpm project sits alongside the existing
  engineering notes with no filename collisions.
- Re-point "Edit on GitHub" links from MHSanaei/3x-ui-docs to this repo's
  docs/content/docs path (docs/lib/shared.ts, docs/app/.../page.tsx).
- Add docs-ci.yml and docs-deploy.yml under .github/workflows/, scoped to
  docs/** and run with working-directory: docs, since GitHub only runs
  workflows from the repo-root .github/. deploy-static.yml's GitHub Pages
  publish (CNAME docs.sanaei.dev) carries over unchanged.

Follow-up (outside this commit): attach the docs.sanaei.dev custom domain to
this repository's Pages (or set the Vercel project's root directory to docs),
confirm the site is live from the monorepo, then delete MHSanaei/3x-ui-docs.

docs/components/tools/shared/fields.tsx

@@ -0,0 +1,99 @@
+'use client';
+
+import { useId } from 'react';
+
+const inputClass =
+  'rounded-lg border bg-fd-background px-3 py-2 text-sm outline-none transition-colors focus-visible:border-fd-primary focus-visible:ring-2 focus-visible:ring-fd-ring/30';
+
+export function TextField({
+  label,
+  value,
+  onChange,
+  placeholder,
+  hint,
+  inputMode,
+}: {
+  label: string;
+  value: string;
+  onChange: (value: string) => void;
+  placeholder?: string;
+  hint?: string;
+  inputMode?: 'numeric' | 'text';
+}) {
+  const id = useId();
+  return (
+    <div className="flex flex-col gap-1.5">
+      <label htmlFor={id} className="text-sm font-medium">
+        {label}
+      </label>
+      {/* Values are technical (hosts, links) and stay LTR even on RTL pages. */}
+      <input
+        id={id}
+        dir="ltr"
+        inputMode={inputMode}
+        value={value}
+        placeholder={placeholder}
+        onChange={(e) => onChange(e.target.value)}
+        className={inputClass}
+      />
+      {hint ? <span className="text-xs text-fd-muted-foreground">{hint}</span> : null}
+    </div>
+  );
+}
+
+export function CheckboxField({
+  label,
+  checked,
+  onChange,
+}: {
+  label: string;
+  checked: boolean;
+  onChange: (checked: boolean) => void;
+}) {
+  const id = useId();
+  return (
+    <label htmlFor={id} className="inline-flex cursor-pointer items-center gap-2 text-sm">
+      <input
+        id={id}
+        type="checkbox"
+        checked={checked}
+        onChange={(e) => onChange(e.target.checked)}
+        className="size-4 accent-fd-primary"
+      />
+      {label}
+    </label>
+  );
+}
+
+export function SelectField({
+  label,
+  value,
+  onChange,
+  options,
+}: {
+  label: string;
+  value: string;
+  onChange: (value: string) => void;
+  options: readonly string[];
+}) {
+  const id = useId();
+  return (
+    <div className="flex flex-col gap-1.5">
+      <label htmlFor={id} className="text-sm font-medium">
+        {label}
+      </label>
+      <select
+        id={id}
+        value={value}
+        onChange={(e) => onChange(e.target.value)}
+        className={inputClass}
+      >
+        {options.map((opt) => (
+          <option key={opt} value={opt}>
+            {opt}
+          </option>
+        ))}
+      </select>
+    </div>
+  );
+}