Add per-course progress badges to library cards, replace alert() with toasts

Compute a cheap, uncached completion percentage per course (completed
lessons / media files) and surface it as a badge + thin progress bar on
library cards in both list and grid view, and in search results - untouched
courses stay unbadged so the list doesn't get noisy with "0%".

Replace the native alert() dialogs used for bulk-select validation and
course-load errors with a small slide-in toast component that matches the
rest of the UI instead of a jarring browser popup.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

offlineu_core.py

@@ -806,6 +806,7 @@ def list_library_directory(dir_path: str, skip_hidden: bool = True) -> Dict[str,
         if _looks_like_course(entry):
             item = _course_summary(entry)
             item['hidden'] = is_hidden
+            item['completion_percentage'] = _course_completion_percentage(entry, item['media_files'])
             items.append(item)
         else:
             if skip_hidden:
@@ -904,6 +905,32 @@ def _course_summary(course_dir: Path) -> Dict[str, Any]:
     return dict(cache_get_or_compute(f'course_summary:{course_dir}', compute))
 
 
+def _course_completion_percentage(course_dir: Path, media_files: int) -> Optional[float]:
+    """
+    Cheap, uncached completion percentage for a single course card - reads
+    only that course's own small progress JSON directly (same pattern as
+    _scan_library_activity), never the cached _course_summary result, so a
+    lesson finished a second ago shows up immediately instead of waiting out
+    the library-scan cache TTL. None if nothing's completed yet, so
+    untouched courses don't show a noisy "0%" badge.
+    """
+    if media_files <= 0:
+        return None
+    try:
+        with open(course_dir / '.offlineu_progress.json', 'r') as f:
+            progress = json.load(f)
+    except (FileNotFoundError, json.JSONDecodeError, OSError):
+        return None
+
+    completed = sum(
+        1 for key, entry in progress.items()
+        if key != 'last_accessed_path' and isinstance(entry, dict) and entry.get('completed')
+    )
+    if completed == 0:
+        return None
+    return round(min(100.0, 100 * completed / media_files), 1)
+
+
 def search_library_courses(query: str) -> List[Dict[str, Any]]:
     """
     Search the library for courses whose name contains `query`
@@ -912,11 +939,14 @@ def search_library_courses(query: str) -> List[Dict[str, Any]]:
     get_all_course_dirs.
     """
     query_lower = query.lower()
-    return [
-        _course_summary(course)
-        for course in get_all_course_dirs()
-        if query_lower in course.name.lower()
-    ]
+    results = []
+    for course in get_all_course_dirs():
+        if query_lower not in course.name.lower():
+            continue
+        item = _course_summary(course)
+        item['completion_percentage'] = _course_completion_percentage(course, item['media_files'])
+        results.append(item)
+    return results
 
 
 def _extract_subtitle_snippet(text: str, query_lower: str, context_chars: int = 80) -> str: