feat(ui): semantic light/dark tokens, ThemeProvider with FOUC guard, token-restyle Toaster/Cover

frontend/index.html

@@ -3,8 +3,17 @@
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
-    <meta name="theme-color" content="#0c0a09" />
+    <meta name="theme-color" content="#fafaf9" />
     <link rel="icon" href="/icon.svg" type="image/svg+xml" />
+    <script>
+      (() => {
+        try {
+          const t = localStorage.getItem("ui.theme") ?? "system";
+          if (t === "dark" || (t === "system" && matchMedia("(prefers-color-scheme: dark)").matches))
+            document.documentElement.classList.add("dark");
+        } catch {}
+      })();
+    </script>
     <title>BookLib — 个人书库</title>
   </head>
   <body>