Add search, thumbnails, continue watching, playback speed, PWA install, sort, notes, and mark-watched

Eight usability additions on top of the mobile redesign: course cover-image
thumbnails, a debounced library search, a Continue Watching section split
from Recently Viewed, per-lesson playback speed control, an installable
PWA manifest/icons, name sort in the library browser, per-lesson notes, and
a bulk mark-course-as-watched action. Also fixes two real bugs found along
the way: lesson_view.html was missing the viewport meta tag (so lesson
pages weren't actually mobile-responsive), and update_lesson_progress
always overwrote the whole progress entry, which would have silently
deleted a saved note on the next routine autosave.

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

templates/help.html

@@ -4,6 +4,9 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>Help - OfflineU</title>
+    <link rel="manifest" href="/static/manifest.json">
+    <meta name="theme-color" content="#007acc">
+    <link rel="apple-touch-icon" href="/static/icons/icon-192.png">
     <style>
         :root {
             --bg-primary: #1a1a1a;