Apply conservative tooltip standard across apps

[jbkunst]

What changed

• Finalizes the repository-wide tooltip standard in README.md, explicitly covering published apps and drafts.
• Removes native HTML title attributes from icon triggers to avoid the second browser tooltip.
• Keeps icon-only triggers accessible with an aria-label wrapper and supports hover focus click.
• Simplifies tooltip theming to Bootstrap defaults plus "tooltip-bg" = "#495057".
• Updates app-template so new apps start with the same tooltip theme.

Repo-wide audit

The canonical apps were reviewed conservatively. Tooltips are added only where they contribute information that is not already obvious or adequately explained in the app README/UI.

Tooltips kept or added

• lorenz-attractor: sigma, rho, beta, and time step. The README shows the equations but does not interpret the controls.
• decision-tree: percent noise only. Depth and alpha are intentionally left without tooltips because the README already explains them.
• pokemon-dimensionality-reduction (Status: draft): t-SNE perplexity plus UMAP neighbors and minimum distance. The README names these parameters but does not explain their effect.

Audited with no new tooltips

kmeans, kmeans-images, arma-process, roc-curve, logistic-regression, underfitting-overfitting, binary-predictions-metrics, network-structures, dimensionality-reduction, and shap-explorer were left unchanged where controls are obvious or the existing README/UI already provides the explanation. Alternative app_*.R files are not treated as canonical app implementations.

Design rule

apptheme <- bs_theme("tooltip-bg" = "#495057")

Apps with existing theme customizations add the same Sass variable to their existing bs_theme() call.

Scope

This PR changes only the shared guidance/template and the apps where the audit found a clear reason for contextual input help. It does not force every app to have tooltips.