feat: 新增键盘快捷键设置

- 设置页新增「键盘快捷键」分类，可自定义快捷键绑定
- 支持快速滚动（Home/End/PageUp/PageDown）与导航跳转（Ctrl/Cmd+数字）
- 快捷键绑定与开关按动作独立持久化，默认关闭
- 新增 ui 包 keybinding 组件与工具（KeybindingModal/KeybindingChips 等）

apps/app-frontend/src/helpers/scroll-top-state.ts

@@ -0,0 +1,20 @@
+const SHOW_SCROLL_TOP_STORAGE_KEY = 'starlight-show-scroll-top'
+const QUICK_SCROLL_ENABLED_STORAGE_KEY = 'starlight-quick-scroll-enabled'
+
+export function getShowScrollTop(): boolean {
+	const value = localStorage.getItem(SHOW_SCROLL_TOP_STORAGE_KEY)
+	return value !== 'false'
+}
+
+export function setShowScrollTop(show: boolean) {
+	localStorage.setItem(SHOW_SCROLL_TOP_STORAGE_KEY, String(show))
+}
+
+export function getQuickScrollEnabled(): boolean {
+	const value = localStorage.getItem(QUICK_SCROLL_ENABLED_STORAGE_KEY)
+	return value === 'true'
+}
+
+export function setQuickScrollEnabled(enabled: boolean) {
+	localStorage.setItem(QUICK_SCROLL_ENABLED_STORAGE_KEY, String(enabled))
+}