fix(web): 文件管理目录树与主面板共用实底 slab 与表头 token(FIX-9)

两块面板垫同一层底色(pg-slab-solid),背后光斑不再让目录树更亮更透;
列表粘性表头改用与卡片同色系的 pg-slab-head,不再是一条纯黑。

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/web/app/components/files/file-list.tsx

@@ -363,7 +363,7 @@ export function FileList({
         <div
           role="row"
           className={cn(
-            "sticky top-0 z-10 grid items-center gap-3 border-b border-border bg-background/95 px-3 py-2 text-meta font-medium text-muted-foreground backdrop-blur",
+            "pg-slab-head sticky top-0 z-10 grid items-center gap-3 border-b border-border px-3 py-2 text-meta font-medium text-muted-foreground",
             DESKTOP_COLS,
           )}
         >