feat: implement profile summary cards with GraphQL fetch and Action wrapper

- Add GraphQL client fetching profile, stats, language aggregation, and
  per-repo commit histograms for the productive-time heatmap.
- Render real SVG cards (profile details, top languages, stats grid,
  weekday×hour heatmap) with XML escaping and thousands-formatted numbers.
- Expand theme palette to 30 built-ins ported from github-readme-stats;
  add -list-themes, multi-theme rendering, and 'all' shortcut.
- Package as Docker-based GitHub Action (action.yml, Dockerfile,
  entrypoint.sh) with optional auto-commit of generated cards.
- Release workflow publishes GHCR image and cross-platform binaries on
  v* tags.
- Unit tests cover rendering, XML escape, number formatting, language sort.

Dockerfile

@@ -0,0 +1,13 @@
+FROM golang:1.26-alpine AS build
+WORKDIR /src
+COPY go.mod ./
+RUN go mod download || true
+COPY . .
+RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /out/ghstats .
+
+FROM alpine:3.21
+RUN apk add --no-cache ca-certificates tzdata git
+COPY --from=build /out/ghstats /usr/local/bin/ghstats
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+ENTRYPOINT ["/entrypoint.sh"]