feat: add React UI for rpm.lair.cafe

- Vite + React + SWC + TypeScript SPA with react-router and
  react-bootstrap
- Dark/light/system theme with Bootstrap 5.3 data-bs-theme
- Home page with repo setup instructions and copyable code blocks
- Package list and detail pages driven by packages.json
- Python script to generate packages.json from repodata XML
- Nginx config updated for SPA fallback, asset caching, removed
  autoindex
- New deploy-ui workflow triggered on ui/ or nginx config changes,
  requires runners with nvm label
- packages.json generation added to publish job after createrepo_c
- Runner setup docs for nvm and sequoia-sq added to readme

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/theme/ThemeToggle.tsx

@@ -0,0 +1,32 @@
+import { Dropdown } from "react-bootstrap";
+import { useTheme } from "./ThemeContext.tsx";
+
+const options = [
+  { key: "light" as const, label: "Light", icon: "☀️" },
+  { key: "dark" as const, label: "Dark", icon: "🌙" },
+  { key: "system" as const, label: "System", icon: "💻" },
+];
+
+export function ThemeToggle() {
+  const { choice, setChoice } = useTheme();
+  const current = options.find((o) => o.key === choice) ?? options[2];
+
+  return (
+    <Dropdown align="end">
+      <Dropdown.Toggle variant="outline-secondary" size="sm">
+        {current.icon}
+      </Dropdown.Toggle>
+      <Dropdown.Menu>
+        {options.map((o) => (
+          <Dropdown.Item
+            key={o.key}
+            active={o.key === choice}
+            onClick={() => setChoice(o.key)}
+          >
+            {o.icon} {o.label}
+          </Dropdown.Item>
+        ))}
+      </Dropdown.Menu>
+    </Dropdown>
+  );
+}