refactor(lyric): 抽取全屏背景/文字颜色逻辑为 useLyricBackground composable

MusicFull.vue 与 MusicFullMobile.vue 各自持有的 setTextColors /
currentBackground / animationFrame / isDark 合并到共享 composable，
消除两份几乎一致的包装逻辑。Mobile 的 --bg-color 差异通过 writeBgColor
option 注入，行为等价。

src/renderer/components/lyric/MusicFullMobile.vue

@@ -408,12 +408,13 @@ import {
   useLyricProgress
 } from '@/hooks/MusicHook';
 import { useArtist } from '@/hooks/useArtist';
+import { useLyricBackground } from '@/hooks/useLyricBackground';
 import { usePlayMode } from '@/hooks/usePlayMode';
 import { audioService } from '@/services/audioService';
 import { usePlayerStore } from '@/store/modules/player';
 import { DEFAULT_LYRIC_CONFIG, LyricConfig } from '@/types/lyric';
 import { getImgUrl, secondToMinute } from '@/utils';
-import { animateGradient, getHoverBackgroundColor, getTextColors } from '@/utils/linearColor';
+import { getTextColors } from '@/utils/linearColor';
 import { showBottomToast } from '@/utils/shortcutToast';
 
 const { t } = useI18n();
@@ -876,10 +877,10 @@ const handleThumbTouchEnd = (e: TouchEvent) => {
   isThumbDragging.value = false;
 };
 
-// 背景相关
-const currentBackground = ref('');
-const animationFrame = ref<number | null>(null);
-const isDark = ref(false);
+// 背景相关（由 composable 管理）
+const { isDark, applyBackground } = useLyricBackground({
+  writeBgColor: () => playerStore.playMusic.primaryColor || undefined
+});
 const config = ref<LyricConfig>({ ...DEFAULT_LYRIC_CONFIG });
 
 // 可见歌词计算
@@ -937,49 +938,6 @@ const isVisible = computed({
   set: (value) => emit('update:modelValue', value)
 });
 
-// 设置文字颜色
-const setTextColors = (background: string) => {
-  if (!background) {
-    textColors.value = getTextColors();
-    document.documentElement.style.setProperty('--hover-bg-color', getHoverBackgroundColor(false));
-    document.documentElement.style.setProperty('--text-color-primary', textColors.value.primary);
-    document.documentElement.style.setProperty('--text-color-active', textColors.value.active);
-    document.documentElement.style.setProperty('--bg-color', 'rgba(25, 25, 25, 1)');
-    return;
-  }
-
-  // 更新文字颜色
-  textColors.value = getTextColors(background);
-  isDark.value = textColors.value.active === '#000000';
-
-  document.documentElement.style.setProperty(
-    '--hover-bg-color',
-    getHoverBackgroundColor(isDark.value)
-  );
-  document.documentElement.style.setProperty('--text-color-primary', textColors.value.primary);
-  document.documentElement.style.setProperty('--text-color-active', textColors.value.active);
-
-  // 解析背景颜色用于封面融合
-  let bgColor = playerStore.playMusic.primaryColor || 'rgba(25, 25, 25, 1)';
-
-  document.documentElement.style.setProperty('--bg-color', bgColor);
-
-  // 处理背景颜色动画
-  if (currentBackground.value) {
-    if (animationFrame.value) {
-      cancelAnimationFrame(animationFrame.value);
-    }
-    const result = animateGradient(currentBackground.value, background, (gradient) => {
-      currentBackground.value = gradient;
-    });
-    if (typeof result === 'number') {
-      animationFrame.value = result;
-    }
-  } else {
-    currentBackground.value = background;
-  }
-};
-
 const targetBackground = computed(() => {
   if (config.value.theme !== 'default') {
     return themeMusic[config.value.theme] || props.background;
@@ -992,17 +950,14 @@ watch(
   targetBackground,
   (newBg) => {
     if (newBg) {
-      setTextColors(newBg);
+      applyBackground(newBg);
     }
   },
   { immediate: true }
 );
 
-// 组件卸载时清理动画
+// 组件卸载清理
 onBeforeUnmount(() => {
-  if (animationFrame.value) {
-    cancelAnimationFrame(animationFrame.value);
-  }
   if (autoScrollTimer.value) {
     clearTimeout(autoScrollTimer.value);
   }
@@ -1113,7 +1068,7 @@ watch(isVisible, (newVal) => {
   if (newVal) {
     // 播放器显示时，重新设置背景颜色
     if (targetBackground.value) {
-      setTextColors(targetBackground.value);
+      applyBackground(targetBackground.value);
     }
   } else {
     showFullLyrics.value = false;