fix(channels): only show the detail-page tag editor on subscribed channels

attach_tag/detach_tag call _user_subscription, which 404s for a channel you don't
follow — but the tag control was rendered for any non-demo channel, so tagging an
un-subscribed/explored channel popped a global error + reverted the optimistic add.
Gate the control on ch.subscribed (personal tags are for your subscriptions anyway).

frontend/src/components/ChannelPage.tsx

@@ -445,7 +445,10 @@ export default function ChannelPage({
                     {t("channel.subscribe")}
                   </button>
                 ))}
-              {ch && !me.is_demo && (
+              {/* Tagging is a subscription-scoped action (attach_tag → _user_subscription 404s
+                  otherwise), and personal tags are for your subscriptions — so only offer it on a
+                  channel you actually follow. */}
+              {ch?.subscribed && !me.is_demo && (
                 <ChannelTagsControl
                   userTags={userTags}
                   attachedIds={ch.tag_ids}