Add a dark mode toggle

Personal, per-browser preference (localStorage), not an admin-wide
Display setting like date/time format — every role can pick their own.
Sets data-bs-theme on <html> to hook into Tabler's built-in dark
variant, applied via an inline script in index.html before React
mounts so there's no light-mode flash on load. Toggle button lives in
the sidebar footer next to Sign out.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/index.html

@@ -4,6 +4,20 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Homelab Manager</title>
+    <script>
+      (function () {
+        try {
+          var stored = localStorage.getItem("hlm-theme");
+          var theme =
+            stored === "light" || stored === "dark"
+              ? stored
+              : window.matchMedia("(prefers-color-scheme: dark)").matches
+                ? "dark"
+                : "light";
+          document.documentElement.setAttribute("data-bs-theme", theme);
+        } catch (e) {}
+      })();
+    </script>
   </head>
   <body>
     <div id="root"></div>