feat(ui): M6.38批 — P1 MD3细化(SDK对话框28)+批②底部上滑回桌面+批6 ncm顶栏回播放页+批④面板后台列表+launcher UI微调(歌词页黑半透明底/播放页布局下移)+封面空artwork保留修复

- P1: meow_ui_dialog.c 对话框底框 radius_card→MEOW_SHAPE_EXTRA_LARGE(28) 三处(:103填充/:104描边/:361 BottomSheet); meow_ui_widgets.h 新增 MEOW_SHAPE_EXTRA_LARGE 28 宏; TextField/FAB/Slider/Progress 按设计文档保留
- 批②: np_gesture.h 新增 NP_GEST_HOME+np_home_gesture(底部热区上滑,与左缘返回对称互斥); now_playing.c 新增 np_draw_home_bar(屏底左缘pill指示条)+手势衔接; test_now_playing_widgets G5b
- 批6: ncm home.c 顶栏加回播放页按钮(MEOW_ICON_QUEUE_MUSIC)→push(ACT_PLAYER), 无音频也生效
- 批④: meowpanel /api/ams/bg 只读端点+前端后台列表页+杀/清复用kill通道
- UI微调: now_playing.c 播放页布局宏下移(NP_COVER_Y 84/NP_BTN_Y 612消底部留白)+歌词页背景统一黑色半透明叠层(弃封面取色底,删 np_draw_lyric_coverbg)
- 封面保留: now_playing.c/notif_center.c 空artwork保留上一张已解码封面(防暂停/开歌单致META报空artwork后封面闪没); test_np_cover C2断言更新+mirror同步

0warning;宿主测试 test_np_cover ALL PASS;launcher/ncm/panel 宿主测试无回归。二进制留批量批次未入库

patch_v2/meowpanel/web/index.html

@@ -69,6 +69,10 @@
         <span class="nav-ico"><svg viewBox="0 0 24 24"><path d="M4 6h16M4 12h16M4 18h16" stroke="currentColor" stroke-width="2" stroke-linecap="round"/><circle cx="9" cy="6" r="2.2" fill="var(--bg)"/><circle cx="15" cy="12" r="2.2" fill="var(--bg)"/><circle cx="7" cy="18" r="2.2" fill="var(--bg)"/></svg></span>
         <span class="nav-label">进程·服务</span>
       </button>
+      <button class="nav-item" data-route="bg"      title="后台活动（App 后台常驻：CPU/内存占用 + 一键清后台）">
+        <span class="nav-ico"><svg viewBox="0 0 24 24"><path d="M12 3 3 8l9 5 9-5-9-5z" fill="none" stroke="currentColor" stroke-width="2" stroke-linejoin="round"/><path d="M3 12l9 5 9-5" fill="none" stroke="currentColor" stroke-width="2" stroke-linejoin="round"/><path d="M3 16l9 5 9-5" fill="none" stroke="currentColor" stroke-width="2" stroke-linejoin="round"/></svg></span>
+        <span class="nav-label">后台</span>
+      </button>
       <button class="nav-item" data-route="system"  title="系统信息 / 升级">
         <span class="nav-ico"><svg viewBox="0 0 24 24"><rect x="6" y="3" width="12" height="18" rx="2" fill="none" stroke="currentColor" stroke-width="2"/><path d="M9 8h6M9 12h6M9 16h3" stroke="currentColor" stroke-width="2" stroke-linecap="round"/></svg></span>
         <span class="nav-label">系统</span>
@@ -92,6 +96,7 @@
       </button>
       <h1 class="topbar-title" id="page-title">首页仪表盘</h1>
       <div class="topbar-actions">
+        <button class="btn topbar-btn" id="bg-clear" hidden>一键清后台</button>
         <span class="conn" id="conn-dot" data-state="unknown" title="后端连接">
           <span class="conn-dot"></span><span class="conn-text" id="conn-text">检测中…</span>
         </span>