80/20 polish: gallery metadata and small UX cleanup

## Goal
Apply a small set of low-effort changes that improve the public experience without redesigning the site or changing the overall architecture.

## Scope
- [ ] Remove the generated `runtime-server` category from the public gallery filters. Runtime is infrastructure, not subject matter.
- [ ] Update the public-site URL in `README.md` to `https://jkunst.com/visual-data-lab/`.
- [ ] Update `app-template/credits.md` to match the standard credits signature documented in the README.
- [ ] Remove `arma-process/Rplots.pdf` and ignore accidental `Rplots.pdf` artifacts.
- [ ] In `roc-curve`, show **Number of observations** only in `Simulated` mode; it has no conceptual role in `Theoretical` mode.
- [ ] In `kmeans-images`, show **Scale to [0, 1]** only when **Use pixel position for clustering** is enabled, where scaling materially changes how RGB and XY dimensions compete.
- [ ] In `arma-process`, remove **Refresh (secs.)** from the main conceptual controls (either hide it under an advanced/display section or use a sensible fixed refresh).

## Principle
Keep the 80/20 rule: do not turn this into a general refactor. Preserve the current theme, layout system, build workflow, metadata validation, Shinylive export and SEO/Open Graph machinery.

## Done when
The public gallery exposes only subject-matter categories, the small root inconsistencies are cleaned up, and the three affected apps have slightly clearer main control surfaces without changing their educational intent.

Start by locating the gallery filter metadata and the entry points for roc-curve, kmeans-images, and arma-process; then inspect README.md, app-template/credits.md, and the existing artifact-ignore rules. Use the repository's metadata validation and export/build workflow to verify the changes. Done means the listed gallery, documentation, artifact, and control-surface updates are complete without altering the site's architecture or educational intent.

Written by the indexing model from the issue text.