style(ui): 表头灰带去除，表格统一白底

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

frontend/src/style.css

@@ -337,7 +337,7 @@ button:not(:disabled), [role="button"]:not([aria-disabled="true"]) { cursor: poi
    over-media tables are untouched; specificity outranks the per-view
    Tailwind hover utilities and the light-mode white rescue. */
 .theme-text .card table thead {
-  background: #fafafa;
+  background: transparent;
   border-bottom: 1px solid rgb(24 24 27 / 0.07);
 }
 /* 表头统一：各页曾分别写 10px/11px、tracking 0.2em/0.04em——收拢到同一规格。 */

frontend/src/views/UsersView.vue

@@ -617,7 +617,7 @@ async function doRecharge() {
   right: 0;
   box-shadow: -10px 0 14px -10px rgb(15 23 42 / 0.16);
 }
-.utable thead th:last-child { background: #fafafa; }
+.utable thead th:last-child { background: var(--surface-solid); }
 .utable tbody td:last-child { background: var(--surface-solid); }
 .utable tbody tr:hover td:last-child { background: #f7f7f8; }
 .utable tbody tr.row-on td:last-child { background: #f1f1f3; }