fix(release): bundle dashboard assets in published wheels and sdist

[pratyush618]

Summary

0.12.0 wheels on PyPI shipped without the compiled dashboard — running taskito dashboard against an installed wheel returns "Dashboard assets not bundled". Two compounding bugs:

1. Maturin packaging. [tool.maturin].python-source = "py_src" only auto-packages .py/.pyi/py.typed — static SPA assets need explicit opt-in via include. Without it, even local maturin build against a built dashboard tree drops the assets.
2. Release pipeline. publish.yml ran maturin build directly across 5 wheel/sdist jobs without ever running pnpm --dir dashboard build first. py_src/taskito/static/ is gitignored, so a clean CI checkout had nothing for maturin to package even before bug (1) silently dropped them.

Fix

• pyproject.toml: explicit include = [{ path = "py_src/taskito/static/dashboard/**/*", format = ["wheel", "sdist"] }] so the dashboard ships in both wheel and sdist.
• .github/actions/dashboard-build/: composite action — single source of truth for pnpm 10.30.3 / Node 22 / install / tsr generate / vite build / output verification. Consumed by both publish.yml (release) and dashboard.yml (CI), so wheels ship the exact build path CI tests.
• .github/workflows/publish.yml: new build-dashboard job builds the dashboard once and uploads as artifact. All 5 wheel/sdist jobs needs: it, download the artifact, and run a test -f index.html verification step before invoking maturin. A missing dashboard now fails CI loudly instead of shipping silently.
• .github/workflows/dashboard.yml: refactored to use the same composite action.
• Version bump 0.12.0 → 0.12.1, changelog entry.

Local verification

• Clean rebuild from scratch (rm -rf py_src/taskito/static/dashboard then full pipeline).
• maturin build --release --features extension-module,postgres,redis,native-async,workflows → wheel contains taskito/static/dashboard/{index.html, assets/*}.
• maturin sdist → tarball contains the dashboard tree.
• Wheel installed into a clean throwaway venv: importlib.resources.files('taskito').joinpath('static/dashboard/index.html').is_file() → True; StaticAssets.from_package() returns a valid object — the runtime probe at py_src/taskito/dashboard/static.py:106-108 that produced the original error now passes.
• cargo check --workspace clean across all 4 crates at 0.12.1.

Test plan

• CI green across the matrix
• Per-job verification step appears in each wheel/sdist job log and asserts index.html exists
• After merging and tagging 0.12.1, inspect a published wheel: unzip -l taskito-0.12.1-*.whl | grep static/dashboard shows index.html + assets