feat: useTheme 훅 + 테마 토글 버튼 구현

- useTheme: localStorage 기반 다크/라이트 테마 전환
- data-theme 속성으로 CSS 변수 자동 전환
- Topbar에 Light/Dark 토글 버튼 추가

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/pages/dashboard/DashboardPage.tsx

@@ -1,5 +1,6 @@
 import { useCallback, useMemo } from "react";
 import { useAuth } from "../../shared/auth";
+import { useTheme } from "../../shared/hooks";
 import { useAisTargetPolling } from "../../features/aisPolling/useAisTargetPolling";
 import type { DerivedLegacyVessel } from "../../features/legacyDashboard/model/types";
 import { LEGACY_ALARM_KINDS } from "../../features/legacyDashboard/model/types";
@@ -58,6 +59,7 @@ function useLegacyIndex(data: LegacyVesselDataset | null) {
 
 export function DashboardPage() {
   const { user, logout } = useAuth();
+  const { theme, toggleTheme } = useTheme();
   const uid = user?.id ?? null;
 
   // ── Data fetching ──
@@ -259,6 +261,8 @@ export function DashboardPage() {
         onLogoClick={onLogoClick}
         userName={user?.name}
         onLogout={logout}
+        theme={theme}
+        onToggleTheme={toggleTheme}
       />
 
       <DashboardSidebar

apps/web/src/shared/hooks/index.ts

@@ -1 +1,2 @@
 export { usePersistedState } from './usePersistedState';
+export { useTheme } from './useTheme';

apps/web/src/shared/hooks/useTheme.ts

@@ -0,0 +1,47 @@
+import { useState, useEffect, useCallback } from 'react';
+
+type Theme = 'dark' | 'light';
+
+const STORAGE_KEY = 'wing:theme';
+const DEFAULT_THEME: Theme = 'dark';
+
+function readTheme(): Theme {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY);
+    if (raw === 'light' || raw === 'dark') return raw;
+  } catch {
+    // storage unavailable
+  }
+  return DEFAULT_THEME;
+}
+
+function applyTheme(theme: Theme) {
+  document.documentElement.dataset.theme = theme;
+}
+
+export function useTheme() {
+  const [theme, setThemeState] = useState<Theme>(() => {
+    const t = readTheme();
+    applyTheme(t);
+    return t;
+  });
+
+  useEffect(() => {
+    applyTheme(theme);
+  }, [theme]);
+
+  const setTheme = useCallback((t: Theme) => {
+    setThemeState(t);
+    try {
+      localStorage.setItem(STORAGE_KEY, t);
+    } catch {
+      // quota exceeded or unavailable
+    }
+  }, []);
+
+  const toggleTheme = useCallback(() => {
+    setTheme(theme === 'dark' ? 'light' : 'dark');
+  }, [theme, setTheme]);
+
+  return { theme, setTheme, toggleTheme } as const;
+}

apps/web/src/widgets/topbar/Topbar.tsx

@@ -11,9 +11,11 @@ type Props = {
   onLogoClick?: () => void;
   userName?: string;
   onLogout?: () => void;
+  theme?: "dark" | "light";
+  onToggleTheme?: () => void;
 };
 
-export function Topbar({ total, fishing, transit, pairLinks, alarms, pollingStatus, lastFetchMinutes, clock, adminMode, onLogoClick, userName, onLogout }: Props) {
+export function Topbar({ total, fishing, transit, pairLinks, alarms, pollingStatus, lastFetchMinutes, clock, adminMode, onLogoClick, userName, onLogout, theme, onToggleTheme }: Props) {
   const statusColor =
     pollingStatus === "ready" ? "#22C55E" : pollingStatus === "loading" ? "#F59E0B" : pollingStatus === "error" ? "#EF4444" : "var(--muted)";
   return (
@@ -55,6 +57,15 @@ export function Topbar({ total, fishing, transit, pairLinks, alarms, pollingStat
         </div>
       </div>
       <div className="ml-2.5 whitespace-nowrap text-[10px] font-semibold text-wing-accent">{clock}</div>
+      {onToggleTheme && (
+        <button
+          className="ml-1 cursor-pointer rounded border border-wing-border bg-transparent px-1.5 py-0.5 text-[9px] text-wing-muted transition-all duration-150 hover:border-wing-accent hover:text-wing-text"
+          onClick={onToggleTheme}
+          title={theme === "dark" ? "라이트 모드로 전환" : "다크 모드로 전환"}
+        >
+          {theme === "dark" ? "Light" : "Dark"}
+        </button>
+      )}
       {userName && (
         <div className="ml-2.5 flex shrink-0 items-center gap-2">
           <span className="whitespace-nowrap text-[10px] font-medium text-wing-text">{userName}</span>