fix(channel): fix shorts/streams pagination using continuation tokens

- Add continuation_token_cache to store ctokens between page requests
- Use cached ctoken for page 2+ instead of generating fresh tokens
- Switch shorts/streams to Next/Previous buttons (no page numbers)
- Show "N+ videos" indicator when more pages are available
- Fix UnboundLocalError when page_call was undefined for shorts/streams

The issue was that YouTube's InnerTube API requires continuation tokens
for pagination on shorts/streams tabs, but the code was generating a new
ctoken each time, always returning the same 30 videos.

youtube/channel.py

@@ -274,6 +274,8 @@ def get_channel_tab(channel_id, page="1", sort=3, tab='videos', view=1,
 
 # cache entries expire after 30 minutes
 number_of_videos_cache = cachetools.TTLCache(128, 30*60)
+# Cache for continuation tokens (shorts/streams pagination)
+continuation_token_cache = cachetools.TTLCache(512, 15*60)
 @cachetools.cached(number_of_videos_cache)
 def get_number_of_videos_channel(channel_id):
     if channel_id is None:
@@ -487,10 +489,46 @@ def get_channel_page_general_url(base_url, tab, request, channel_id=None):
         if not channel_id:
             channel_id = get_channel_id(base_url)
 
-        # Use youtubei browse API with continuation token for all pages
-        page_call = (get_channel_tab, channel_id, str(page_number), sort,
-                     tab, int(view))
-        continuation = True
+        # For shorts/streams, use continuation token from cache or request
+        if tab in ('shorts', 'streams'):
+            if ctoken:
+                # Use ctoken directly from request (passed via pagination)
+                polymer_json = util.call_youtube_api('web', 'browse', {
+                    'continuation': ctoken,
+                })
+                continuation = True
+            elif page_number > 1:
+                # For page 2+, get ctoken from cache
+                cache_key = (channel_id, tab, sort, page_number - 1)
+                cached_ctoken = continuation_token_cache.get(cache_key)
+                if cached_ctoken:
+                    polymer_json = util.call_youtube_api('web', 'browse', {
+                        'continuation': cached_ctoken,
+                    })
+                    continuation = True
+                else:
+                    # Fallback: generate fresh ctoken
+                    page_call = (get_channel_tab, channel_id, str(page_number), sort, tab, int(view))
+                    continuation = True
+                    polymer_json = gevent.spawn(*page_call)
+                    polymer_json.join()
+                    if polymer_json.exception:
+                        raise polymer_json.exception
+                    polymer_json = polymer_json.value
+            else:
+                # Page 1: generate fresh ctoken
+                page_call = (get_channel_tab, channel_id, str(page_number), sort, tab, int(view))
+                continuation = True
+                polymer_json = gevent.spawn(*page_call)
+                polymer_json.join()
+                if polymer_json.exception:
+                    raise polymer_json.exception
+                polymer_json = polymer_json.value
+        else:
+            # videos tab - original logic
+            page_call = (get_channel_tab, channel_id, str(page_number), sort,
+                         tab, int(view))
+            continuation = True
 
         if tab == 'videos':
             # Only need video count for the videos tab
@@ -505,14 +543,7 @@ def get_channel_page_general_url(base_url, tab, request, channel_id=None):
             gevent.joinall(tasks)
             util.check_gevent_exceptions(*tasks)
             number_of_videos, polymer_json = tasks[0].value, tasks[1].value
-        else:
-            # For shorts/streams, item count is used instead
-            polymer_json = gevent.spawn(*page_call)
-            polymer_json.join()
-            if polymer_json.exception:
-                raise polymer_json.exception
-            polymer_json = polymer_json.value
-            number_of_videos = 0  # will be replaced by actual item count later
+        # For shorts/streams, polymer_json is already set above, nothing to do here
 
     elif tab == 'about':
         # polymer_json = util.fetch_url(base_url + '/about?pbj=1', headers_desktop, debug_name='gen_channel_about')
@@ -580,9 +611,13 @@ def get_channel_page_general_url(base_url, tab, request, channel_id=None):
 
     if tab in ('videos', 'shorts', 'streams'):
         if tab in ('shorts', 'streams'):
-            # For shorts/streams, use the actual item count since
-            # get_number_of_videos_channel counts regular uploads only
+            # For shorts/streams, use ctoken to determine pagination
+            info['is_last_page'] = (info.get('ctoken') is None)
             number_of_videos = len(info.get('items', []))
+            # Cache the ctoken for next page
+            if info.get('ctoken'):
+                cache_key = (channel_id, tab, sort, page_number)
+                continuation_token_cache[cache_key] = info['ctoken']
         info['number_of_videos'] = number_of_videos
         info['number_of_pages'] = math.ceil(number_of_videos/page_size) if number_of_videos else 1
         info['header_playlist_names'] = local_playlist.get_playlist_names()