fix(theme): sync SongUI tokens and brighten dark

src/styles/song-theme.css

@@ -39,6 +39,9 @@
 
   --song-seal: #a84635;
 
+  /* 底纹线条（与底色低对比） */
+  --song-scrim-line: rgba(90, 107, 116, 0.14);
+
   --song-shadow-1: 0 1px 2px rgba(20, 24, 27, 0.06);
   --song-shadow-2: 0 2px 8px rgba(20, 24, 27, 0.08);
   --song-shadow-3: 0 8px 24px rgba(20, 24, 27, 0.12);
@@ -68,11 +71,11 @@
 :root[data-theme='dark'] {
   color-scheme: dark;
   --song-bg: #14181b;
-  --song-bg-elevated: #1d2326;
-  --song-surface-3: #232b2f;
+  --song-bg-elevated: #20272b;
+  --song-surface-3: #273036;
 
   --song-text: #e6ecef;
-  --song-text-secondary: #a9b6be;
+  --song-text-secondary: #b9c6cd;
   --song-text-disabled: #5e6a71;
 
   --song-primary: #9dbeee;
@@ -99,6 +102,9 @@
 
   --song-seal: #c76a4f;
 
+  /* 底纹线条（暗色下为淡月白，勿用深色线否则整页显暗） */
+  --song-scrim-line: rgba(230, 236, 239, 0.1);
+
   --song-shadow-1: 0 1px 2px rgba(0, 0, 0, 0.3);
   --song-shadow-2: 0 2px 8px rgba(0, 0, 0, 0.35);
   --song-shadow-3: 0 8px 24px rgba(0, 0, 0, 0.45);