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>

backend/services/ytdlp.py

@@ -82,6 +82,19 @@ def _normalize_video(info: dict) -> dict:
     }
 
 
+def _channel_banner(thumbnails: list | None) -> str | None:
+    if not thumbnails:
+        return None
+    for t in thumbnails:
+        if "banner" in str(t.get("id") or "").lower():
+            return t.get("url")
+    wide = [t for t in thumbnails
+            if t.get("width") and t.get("height") and t["width"] > t["height"] * 3]
+    if wide:
+        return max(wide, key=lambda t: t.get("width") or 0).get("url")
+    return None
+
+
 def _channel_avatar(thumbnails: list | None) -> str | None:
     """Pick the channel avatar from yt-dlp's thumbnails list.
 
@@ -109,7 +122,7 @@ def _normalize_channel(info: dict) -> dict:
         "name": info.get("channel") or info.get("title") or info.get("uploader") or None,
         "description": info.get("description") or None,
         "thumbnail_url": _channel_avatar(info.get("thumbnails")),
-        "banner_url": None,
+        "banner_url": _channel_banner(info.get("thumbnails")),
         "subscriber_count": info.get("channel_follower_count"),
     }