Move build system from setuptools to uv, wire uv into CI/CD

Convert both pyproject.toml files to the uv_build backend and turn the
repo into a proper uv workspace (plotstyle moved to src layout, since
uv_build requires the module in its own subdirectory). Dev tooling
moves from an optional-dependencies extra to a dependency-group, and
plotstyle's matplotlib dependency is now its own rather than a
gallery "plotting" extra.

Gitea Actions workflows and the Dockerfile now use astral-sh/setup-uv,
uv sync, uv run, uv build, and uv publish throughout instead of pip,
build, and twine. Also fixes .dockerignore, which was excluding
uv.lock, deploy/, and README.md and would have broken even the
previous Dockerfile's COPY of deploy/entrypoint.sh.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Dockerfile

@@ -1,3 +1,15 @@
+FROM python:3.11-slim AS builder
+
+COPY --from=ghcr.io/astral-sh/uv:latest /uv /uvx /usr/local/bin/
+
+WORKDIR /app
+
+COPY pyproject.toml uv.lock README.md ./
+COPY plotstyle/pyproject.toml plotstyle/README.md plotstyle/
+COPY gallery/ gallery/
+
+RUN uv build --package gallery -o dist
+
 FROM python:3.11-slim
 
 RUN apt-get update \
@@ -6,12 +18,10 @@ RUN apt-get update \
 
 WORKDIR /app
 
-COPY pyproject.toml .
-COPY gallery/ gallery/
-COPY tests/ tests/
+COPY --from=builder /app/dist/*.whl /tmp/
 COPY deploy/entrypoint.sh deploy/entrypoint.sh
 
-RUN pip install --no-cache-dir ".[dev]"
+RUN pip install --no-cache-dir /tmp/*.whl && rm /tmp/*.whl
 
 RUN useradd -r -s /bin/false gallery
 USER gallery