feat(filters): show channel counts on topic and language chips

The per-tag channel_count was already returned by GET /api/tags but only
surfaced in the chip tooltip. Render it as a small count badge on the chip
face so the relative weight of each topic/language is visible at a glance.

frontend/src/components/Sidebar.tsx

@@ -81,13 +81,20 @@ function TagChip({
     <button
       onClick={onClick}
       title={t("sidebar.channelCount", { count: tag.channel_count })}
-      className={`text-xs px-2.5 py-1 rounded-full border shadow-sm hover:shadow active:translate-y-px transition ${
+      className={`inline-flex items-center gap-1.5 text-xs px-2.5 py-1 rounded-full border shadow-sm hover:shadow active:translate-y-px transition ${
         active
           ? "bg-accent text-accent-fg border-accent"
           : "bg-card border-border text-fg hover:border-accent"
       }`}
     >
-      {tag.name}
+      <span>{tag.name}</span>
+      <span
+        className={`tabular-nums text-[10px] leading-none px-1 py-0.5 rounded-full ${
+          active ? "bg-accent-fg/20 text-accent-fg" : "bg-muted/15 text-muted"
+        }`}
+      >
+        {tag.channel_count}
+      </span>
     </button>
   );
 }