feat: 新增启动器界面缩放设置

在高缩放/小屏下，内嵌网页（如皮肤站）顶栏会被裁掉。新增“界面
与外观”中的“启动器缩放”设置，可整体缩放启动器界面。

- 新增 helpers/ui-scale.ts：用 localStorage 保存缩放值（纯本地
  显示偏好，无需后端），并通过 CSS zoom 应用缩放（含内嵌网页）。
- App.vue 启动时调用 installUiScale() 应用已保存的缩放。
- AppearanceSettings.vue 新增缩放设置项：100/110/125/150 预设
  加 50–200 滑条自定义。

apps/app-frontend/src/components/ui/settings/AppearanceSettings.vue

@@ -26,6 +26,7 @@ import { computed, ref, watch } from 'vue'
 
 import { get, set } from '@/helpers/settings.ts'
 import { getOS } from '@/helpers/utils'
+import { setUiScale, uiScale } from '@/helpers/ui-scale'
 import { useTheming } from '@/store/state'
 import {
 	type AccentColor,
@@ -209,6 +210,23 @@ const messages = defineMessages({
 		defaultMessage:
 			'Enables advanced rendering such as blur effects that may cause performance issues without hardware-accelerated rendering.',
 	},
+	uiScaleTitle: {
+		id: 'app.appearance-settings.ui-scale.title',
+		defaultMessage: 'Launcher scale',
+	},
+	uiScaleDescription: {
+		id: 'app.appearance-settings.ui-scale.description',
+		defaultMessage:
+			'Scale the whole launcher interface. A smaller scale fits more content, which helps when embedded pages are cut off.',
+	},
+	uiScaleCustom: {
+		id: 'app.appearance-settings.ui-scale.custom',
+		defaultMessage: 'Custom',
+	},
+	uiScalePercent: {
+		id: 'app.appearance-settings.ui-scale.percent',
+		defaultMessage: '{value}%',
+	},
 	pageTransitionsTitle: {
 		id: 'app.appearance-settings.page-transitions.title',
 		defaultMessage: 'Page transition animations',
@@ -1020,6 +1038,43 @@ watch(
 					/>
 				</template>
 			</SettingsRow>
+			<SettingsRow stacked>
+				<template #label>
+					<span id="settings-target-appearance-ui-scale" tabindex="-1">
+						{{ formatMessage(messages.uiScaleTitle) }}
+					</span>
+				</template>
+				<template #description>{{ formatMessage(messages.uiScaleDescription) }}</template>
+				<template #control>
+					<div class="flex w-full flex-col gap-3">
+						<div class="flex flex-wrap gap-2">
+							<button
+								v-for="preset in [100, 110, 125, 150]"
+								:key="preset"
+								type="button"
+								class="min-w-16 rounded-lg border border-solid px-3 py-1.5 text-sm font-semibold transition-colors"
+								:class="
+									uiScale === preset
+										? 'border-brand bg-brand-highlight text-brand'
+										: 'border-surface-4 bg-surface-3 text-secondary hover:border-surface-5 hover:text-contrast'
+								"
+								@click="setUiScale(preset)"
+							>
+								{{ preset }}%
+							</button>
+						</div>
+						<Slider
+							id="ui-scale"
+							:model-value="uiScale"
+							:min="50"
+							:max="200"
+							:step="5"
+							unit="%"
+							@update:model-value="(value) => setUiScale(Number(value))"
+						/>
+					</div>
+				</template>
+			</SettingsRow>
 			<SettingsRow>
 				<template #label>
 					<span id="settings-target-appearance-page-transitions" tabindex="-1">