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/static/js/common.js

@@ -114,3 +114,60 @@ function copyTextToClipboard(text) {
 window.addEventListener('DOMContentLoaded', function() {
     cur_track_idx = getDefaultTranscriptTrackIdx();
 });
+
+/**
+ * Thumbnail fallback handler
+ * Tries lower quality thumbnails when higher quality fails (404)
+ * Priority: hq720.jpg -> sddefault.jpg -> hqdefault.jpg -> mqdefault.jpg -> default.jpg
+ */
+function thumbnail_fallback(img) {
+    const src = img.src || img.dataset.src;
+    if (!src) return;
+
+    // Handle YouTube video thumbnails
+    if (src.includes('/i.ytimg.com/')) {
+        // Extract video ID from URL
+        const match = src.match(/\/vi\/([^/]+)/);
+        if (!match) return;
+
+        const videoId = match[1];
+        const imgPrefix = settings_img_prefix || '';
+
+        // Define fallback order (from highest to lowest quality)
+        const fallbacks = [
+            'hq720.jpg',
+            'sddefault.jpg',
+            'hqdefault.jpg',
+            'mqdefault.jpg',
+            'default.jpg'
+        ];
+
+        // Find current quality and try next fallback
+        for (let i = 0; i < fallbacks.length; i++) {
+            if (src.includes(fallbacks[i])) {
+                // Try next quality
+                if (i < fallbacks.length - 1) {
+                    const newSrc = imgPrefix + 'https://i.ytimg.com/vi/' + videoId + '/' + fallbacks[i + 1];
+                    if (img.dataset.src) {
+                        img.dataset.src = newSrc;
+                    } else {
+                        img.src = newSrc;
+                    }
+                }
+                break;
+            }
+        }
+    }
+    // Handle YouTube channel avatars (ggpht.com)
+    else if (src.includes('ggpht.com') || src.includes('yt3.ggpht.com')) {
+        // Try to increase avatar size (s88 -> s240)
+        const newSrc = src.replace(/=s\d+-c-k/, '=s240-c-k-c0x00ffffff-no-rj');
+        if (newSrc !== src) {
+            if (img.dataset.src) {
+                img.dataset.src = newSrc;
+            } else {
+                img.src = newSrc;
+            }
+        }
+    }
+}