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/settings.html

@@ -4,6 +4,9 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>Settings - 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;
@@ -453,6 +456,16 @@
                 </label>
                 <button class="btn btn-secondary" onclick="resetVideoSize()">Reset size</button>
             </div>
+            <div class="setting-row">
+                <label for="playback_speed">Default playback speed
+                    <span class="setting-desc">Applied when a lesson loads; still adjustable per-lesson</span>
+                </label>
+                <select id="playback_speed" data-setting="playback_speed">
+                    {% for speed in ['0.75', '1', '1.25', '1.5', '1.75', '2'] %}
+                    <option value="{{ speed }}" {% if settings.playback_speed == speed %}selected{% endif %}>{{ speed }}x</option>
+                    {% endfor %}
+                </select>
+            </div>
         </div>
 
         <div class="actions">