feat(appearance): add AppearanceProvider and use-appearance hooks

新增 React Context 管理 themeColor 和 zoomLevel state，与 next-themes 正交，
通过 localStorage 持久化并支持跨标签页同步。提供语义化 hook
useThemeColor / useZoomLevel 供调用点按需使用。

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

src/hooks/use-appearance.ts

@@ -0,0 +1,24 @@
+"use client"
+
+import { useContext } from "react"
+import { AppearanceContext } from "@/components/appearance-provider"
+
+export function useAppearance() {
+  const ctx = useContext(AppearanceContext)
+  if (!ctx) {
+    throw new Error("useAppearance must be used within AppearanceProvider")
+  }
+  return ctx
+}
+
+/** 语义化包装：只关心主题色的调用点用这个 */
+export function useThemeColor() {
+  const { themeColor, setThemeColor } = useAppearance()
+  return { themeColor, setThemeColor }
+}
+
+/** 语义化包装：只关心缩放档位的调用点用这个 */
+export function useZoomLevel() {
+  const { zoomLevel, setZoomLevel } = useAppearance()
+  return { zoomLevel, setZoomLevel }
+}