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>

plotstyle/CLAUDE.md

@@ -21,15 +21,18 @@ produces. Everything below is the reference; the notebook is the demo.
 
 ## Install
 
+`plotstyle` is its own project (`plotstyle/pyproject.toml`, `uv_build`
+backend, code under `plotstyle/src/plotstyle/`) and a member of this repo's
+uv workspace (`[tool.uv.workspace]` in the root `pyproject.toml`) — not a
+`gallery` extra.
+
 ```bash
-uv sync --extra plotting          # this repo's uv workflow
-# or
-pip install -e ".[plotting]"
+uv sync --all-packages            # this repo's uv workflow — installs gallery + plotstyle together
 ```
 
-`plotstyle` is an optional extra (`matplotlib>=3.7`) so core `gallery`
-installs stay lightweight — don't add matplotlib to `gallery`'s own
-unconditional dependencies to support this package.
+`matplotlib>=3.7` is `plotstyle`'s own direct dependency — don't add it to
+`gallery`'s dependencies to support this package; `gallery` should stay
+installable without ever pulling in matplotlib.
 
 **Hard requirement: a working local LaTeX toolchain (`latex` + `dvipng`).**
 `plotstyle.use()` sets `text.usetex = True` unconditionally — there is no