Add theme preset UI and Docker compose

- /edit/theme-settings now lists user-defined presets with preview
  swatches, edit and delete actions
- Add/edit preset form at /edit/theme-settings/presets/new and /{key}
- Built-in catalog (14 themes from docs/themes.md) with one-click import
- docker-compose.yml using ghcr.io/uhlwoogi/modern-glance:latest
- Dockerfile gains OCI source label for GHCR repo linking

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,17 @@
+services:
+  glance:
+    image: ghcr.io/uhlwoogi/modern-glance:latest
+    ports:
+      - "8080:8080"
+    volumes:
+      # Config directory: glance.yml lives here and is written by the /edit UI.
+      # Copy docs/glance.yml → ./config/glance.yml to get started.
+      - ./config:/app/config
+      # Assets directory: custom CSS, images, icons referenced by server.assets-path.
+      # Set `server: { assets-path: /app/assets }` in glance.yml to enable.
+      - ./assets:/app/assets
+    restart: unless-stopped
+    # Uncomment to inject secrets as environment variables.
+    # Reference them in glance.yml as ${env:MY_SECRET_TOKEN}.
+    # environment:
+    #   MY_SECRET_TOKEN: abc123