Release v0.4.0 - HD Thumbnails, YouTube 2024+ Support, and yt-dlp Integration

Major Features:
- HD video thumbnails (hq720.jpg) with automatic fallback to lower qualities
- HD channel avatars (240x240 instead of 88x88)
- YouTube 2024+ lockupViewModel support for channel playlists
- youtubei/v1/browse API integration for channel playlist tabs
- yt-dlp integration for multi-language audio and subtitles

Bug Fixes:
- Fixed undefined `abort` import in playlist.py
- Fixed undefined functions in proto.py (encode_varint, bytes_to_hex, succinct_encode)
- Fixed missing `traceback` import in proto_debug.py
- Fixed blurry playlist thumbnails using default.jpg instead of HD versions
- Fixed channel playlists page using deprecated pbj=1 format

Improvements:
- Automatic thumbnail fallback system (hq720 → sddefault → hqdefault → mqdefault → default)
- JavaScript thumbnail_fallback() handler for 404 errors
- Better thumbnail quality across all pages (watch, channel, playlist, subscriptions)
- Consistent HD avatar display for all channel items
- Settings system automatically adds new settings without breaking user config

Files Modified:
- youtube/watch.py - HD thumbnails for related videos and playlist items
- youtube/channel.py - HD thumbnails for channel playlists, youtubei API integration
- youtube/playlist.py - HD thumbnails, fixed abort import
- youtube/util.py - HD thumbnail URLs, avatar HD upgrade, prefix_url improvements
- youtube/comments.py - HD video thumbnail
- youtube/subscriptions.py - HD thumbnails, fixed abort import
- youtube/yt_data_extract/common.py - lockupViewModel support, extract_lockup_view_model_info()
- youtube/yt_data_extract/everything_else.py - HD playlist thumbnails
- youtube/proto.py - Fixed undefined function references
- youtube/proto_debug.py - Added traceback import
- youtube/static/js/common.js - thumbnail_fallback() handler
- youtube/templates/*.html - Added onerror handlers for thumbnail fallback
- youtube/version.py - Bump to v0.4.0

Technical Details:
- All thumbnail URLs now use hq720.jpg (1280x720) when available
- Fallback handled client-side via JavaScript onerror handler
- Server-side avatar upgrade via regex in util.prefix_url()
- lockupViewModel parser extracts contentType, metadata, and first_video_id
- Channel playlist tabs now use youtubei/v1/browse instead of deprecated pbj=1
- Settings version system ensures backward compatibility

youtube/__init__.py

@@ -137,9 +137,22 @@ def error_page(e):
             error_message += '\n\nExit node IP address: ' + exc_info()[1].ip
         return flask.render_template('error.html', error_message=error_message, slim=slim), 502
     elif exc_info()[0] == util.FetchError and exc_info()[1].error_message:
+        # Handle specific error codes with user-friendly messages
+        error_code = exc_info()[1].code
+        error_msg = exc_info()[1].error_message
+
+        if error_code == '400':
+            error_message = (f'Error: Bad Request (400)\n\n{error_msg}\n\n'
+                           'This usually means the URL or parameters are invalid. '
+                           'Try going back and trying a different option.')
+        elif error_code == '404':
+            error_message = 'Error: The page you are looking for isn\'t here.'
+        else:
+            error_message = f'Error: {error_code} - {error_msg}'
+
         return (flask.render_template(
                     'error.html',
-                    error_message=exc_info()[1].error_message,
+                    error_message=error_message,
                     slim=slim
                 ), 502)
     elif (exc_info()[0] == util.FetchError