fix(ui): cleaner active-tab indicator (drop odd v4 slider) (#1480)

Vuetify 4's MD3 v-tab slider underline rendered wider than the tab and floated
below it (operator-flagged in the v4 review). The active tab's text is already
accent-coloured, so drop the slider and mark the active tab with a subtle accent
fill + rounded top — a clean highlight, app-wide across all tabbed views.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/styles/app.css

@@ -49,3 +49,15 @@
   ul, ol, figure, details, summary { padding: 0; margin: 0; }
   h1, h2, h3, h4, h5, h6, p { margin: 0; }
 }
+
+/* Active-tab indicator (operator-flagged 2026-07-13 in the Vuetify-4 review): v4's
+   MD3 v-tab "slider" underline renders wider than the tab and floats below it. The
+   active tab's TEXT is already accent-coloured (color="accent"), so drop the slider
+   and mark the active tab with a subtle accent fill + rounded top — a clean,
+   unambiguous highlight app-wide (Subscriptions / Browse / Settings / Series). */
+.v-tab__slider { display: none !important; }
+.v-tab[aria-selected="true"],
+.v-tab.v-tab--selected {
+  background: rgb(var(--v-theme-accent) / 0.12);
+  border-radius: 8px 8px 0 0;
+}