frontend: rebuild bookmark page with drag-and-drop, search, and theme system

- bookmark management with dnd-kit reordering, bulk edit, search,
  category filter/rename, and meta auto-fetch
- migrate /bookmarks → /dashboard/bookmarks under (main) layout
- homepage redesign with category grid, /-key search, dock tooltips
- theme toggle + use-theme, sonner toasts, alert-dialog/skeleton,
  visual refresh of auth pages

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/lib/use-theme.ts

@@ -0,0 +1,56 @@
+"use client";
+
+import { useEffect, useState } from "react";
+
+type Theme = "light" | "dark";
+
+function readTheme(): Theme {
+  if (typeof window === "undefined") return "light";
+  const stored = localStorage.getItem("theme");
+  if (stored === "light" || stored === "dark") return stored;
+  return window.matchMedia("(prefers-color-scheme: dark)").matches
+    ? "dark"
+    : "light";
+}
+
+function applyTheme(theme: Theme) {
+  const root = document.documentElement;
+  if (theme === "dark") root.classList.add("dark");
+  else root.classList.remove("dark");
+}
+
+export function useTheme() {
+  const [theme, setThemeState] = useState<Theme>("light");
+  const [mounted, setMounted] = useState(false);
+
+  useEffect(() => {
+    setThemeState(readTheme());
+    setMounted(true);
+  }, []);
+
+  useEffect(() => {
+    if (typeof window === "undefined") return;
+    const onStorage = (e: StorageEvent) => {
+      if (e.key === "theme" && (e.newValue === "light" || e.newValue === "dark")) {
+        setThemeState(e.newValue);
+        applyTheme(e.newValue);
+      }
+    };
+    window.addEventListener("storage", onStorage);
+    return () => window.removeEventListener("storage", onStorage);
+  }, []);
+
+  function setTheme(next: Theme) {
+    setThemeState(next);
+    try {
+      localStorage.setItem("theme", next);
+    } catch {}
+    applyTheme(next);
+  }
+
+  function toggle() {
+    setTheme(theme === "dark" ? "light" : "dark");
+  }
+
+  return { theme, setTheme, toggle, mounted };
+}