Shell: scope nav-item link styles under .tac-sidebar (beats host .prose a)

Gradio's .prose a re-coloured/underlined the nav links; bumping specificity with
the .tac-sidebar ancestor (framework-neutral) keeps the sidebar's own styling.

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

web/shell/sidebar.css

@@ -49,15 +49,17 @@ body:not(.tac-collapsed) .gradio-container { margin-left: var(--tac-w); }
   font-size: 10px; letter-spacing: .18em; text-transform: uppercase;
   color: var(--tac-muted); padding: 4px 8px;
 }
-.tac-nav-item {
+/* `.tac-sidebar` ancestor raises specificity so host link styles (e.g. Gradio's
+ * `.prose a`) don't re-colour/underline nav items — framework-neutral. */
+.tac-sidebar .tac-nav-item {
   display: flex; align-items: center; gap: 8px; width: 100%;
   padding: 8px 10px; margin: 1px 0; border: 0; border-radius: 8px;
   background: none; color: var(--tac-ink); font: inherit; text-align: left;
   cursor: pointer; text-decoration: none;
 }
-.tac-nav-item:hover { background: var(--tac-bg-2); }
-.tac-nav-item.active { background: color-mix(in srgb, var(--tac-accent) 22%, transparent); }
-.tac-nav-item .tac-ico { width: 18px; text-align: center; opacity: .9; }
+.tac-sidebar .tac-nav-item:hover { background: var(--tac-bg-2); text-decoration: none; }
+.tac-sidebar .tac-nav-item.active { background: color-mix(in srgb, var(--tac-accent) 22%, transparent); color: var(--tac-ink); }
+.tac-sidebar .tac-nav-item .tac-ico { width: 18px; text-align: center; opacity: .9; }
 
 /* Edge tab to reopen when collapsed. */
 .tac-reopen {