feat(results): average, median, range and Net Promoter Score for scale questions

[global-prog]

Summary

Scale questions currently show a bar per value, which makes it hard to compare two questions, or two runs of the same form, at a glance.

This adds a small set of figures above the existing bars for linearscale questions:

• count, average, median, lowest, highest
• for a 0–10 scale, a Net Promoter Score with its promoter and detractor shares (the usual 9–10 / 7–8 / 0–6 split)

The median sits next to the average deliberately — at the response counts most forms see, one extreme answer moves an average a long way, and showing both makes that visible.

The NPS line only appears when the scale is actually 0–10, since that is the shape those scales are almost always used for.

Scope

Purely additive and presentational:

• one file changed, src/components/Results/ResultsSummary.vue
• no API change, no schema change, no migration
• questions of other types render exactly as before
• the existing per-value bars are untouched; the figures are shown above them

Testing

• npm run lint and npm run stylelint clean
• built against current main (vite --mode production build) with no errors
• checked with an empty result set (renders nothing), a single response, and a 0–10 scale

Happy to adjust the wording, the choice of figures, or the placement if you would prefer them elsewhere in the summary.