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>

static/manifest.json

@@ -0,0 +1,23 @@
+{
+  "name": "OfflineU",
+  "short_name": "OfflineU",
+  "description": "Your self-hosted course library",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#1a1a1a",
+  "theme_color": "#007acc",
+  "icons": [
+    {
+      "src": "/static/icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/static/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ]
+}