Add live demo links to READMEs, enable theme switching in Gallery app, and update deployment workflow for web support.

.github/workflows/deploy-gallery-pages.yml

@@ -31,11 +31,17 @@ jobs:
           channel: "stable"
 
       - name: Flutter version
+        working-directory: apps/gallery
         run: flutter --version
 
       - name: Enable web
+        working-directory: apps/gallery
         run: flutter config --enable-web
 
+      - name: Create web
+        working-directory: apps/gallery
+        run: flutter create . --platforms web --overwrite
+
       - name: Pub get
         working-directory: apps/gallery
         run: flutter pub get