Add colored grid-card fallbacks, bottom tab bar, dashboard reorder

Three visual/structural changes:

- Grid cards without a thumbnail (nearly all real courses in this
  library) showed the same generic graduation-cap icon, making the
  poster grid hard to visually scan. Replace it with a deterministic
  color+initial per course name (same idea as Slack/Google Photos
  default avatars), so cards without real cover art still look
  distinct from each other.

- Notes/Help/Settings only existed as small footer text links,
  meaning a scroll to the bottom of every page to navigate. Add a
  persistent bottom tab bar (Home/Notes/Help/Settings, current page
  highlighted via a new active_tab context var) to the dashboard,
  Notes Hub, Settings, and Help pages. Deliberately left the lesson
  page alone - it already has its own fixed-bottom notes panel, and a
  second fixed bar there would just eat into an already content-dense
  page.

- The no-course dashboard stacked five informational/queue cards
  (Library Stats, Next Up, Pick Back Up, Recently Added, Continue
  Watching, Recently Viewed) before "Your Courses," so the actual
  library browser was the last thing on the page. Move it up to right
  after the Surprise Me banner - it was already unconditional, so this
  only changes where it renders, not when.

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

offlineu_core.py

@@ -1833,7 +1833,8 @@ def index():
                                library_stats=format_library_stats(scan),
                                stale_courses=format_stale_courses(scan),
                                activity_heatmap=format_activity_heatmap(scan),
-                               next_up=get_next_up_courses())
+                               next_up=get_next_up_courses(),
+                               active_tab='home')
 
     # Apply progress data to tree
     ProgressTracker.apply_progress_to_tree(current_course)
@@ -1854,7 +1855,8 @@ def index():
                            recent_views=recent_views,
                            has_note=bool(course_has_any_notes(current_course)),
                            is_queued=os.path.abspath(current_course.path) in get_next_up_paths(),
-                           resume_lesson=resume_lesson)
+                           resume_lesson=resume_lesson,
+                           active_tab='home')
 
 
 @app.route('/browse')
@@ -2252,6 +2254,7 @@ def settings_page():
         density_choices=['comfortable', 'compact'],
         card_style_choices=['flat', 'elevated', 'bordered'],
         corner_radius_choices=['sharp', 'rounded', 'pill'],
+        active_tab='settings',
     )
 
 
@@ -2383,13 +2386,13 @@ def open_recent():
 @app.route('/help')
 def help_page():
     """Static help page: how to use the app, supported file types."""
-    return render_template('help.html')
+    return render_template('help.html', active_tab='help')
 
 
 @app.route('/notes')
 def notes_hub():
     """Every lesson note across the whole library in one place."""
-    return render_template('notes_hub.html', notes=get_all_notes())
+    return render_template('notes_hub.html', notes=get_all_notes(), active_tab='notes')
 
 
 @app.route('/library/search-transcripts')