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>

Dockerfile

@@ -6,6 +6,8 @@ RUN CGO_ENABLED=0 go build .
 
 FROM alpine:3.21
 
+LABEL org.opencontainers.image.source=https://github.com/uhlwoogi/modern-glance
+
 WORKDIR /app
 COPY --from=builder /app/glance .