Mobile nav, channel banners, search channel linking

- Add bottom tab bar (Home/Following/Discover/Downloads/Settings) for mobile
- Fetch and display channel banner images on channel pages
- Fix ChannelCard: channels without a local DB id now follow+navigate on click

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/Channel.jsx

@@ -79,6 +79,13 @@ export default function ChannelPage() {
 
   return (
     <div className="flex flex-col gap-8">
+      {/* Banner */}
+      {channel.banner_url && (
+        <div className="-mx-4 -mt-6 mb-2">
+          <img src={channel.banner_url} alt="" className="w-full h-28 sm:h-44 object-cover" />
+        </div>
+      )}
+
       {/* Channel header */}
       <div className="flex items-start gap-5">
         {channel.thumbnail_url ? (