Add stat display type to GLQL

What does this MR do and why?

Adds a stat display type to GLQL, which renders a single aggregated metric as a large value — the second of the SDLC dashboard visualization types.

• New StatPresenter (stat.vue) renders @gitlab/ui's GlSingleStat with an empty title , so the presenter shows only the value. The title and subtitle (e.g. the time range) are owned by the surrounding container — the dashboard panel or GLQL embed — mirroring the analytics dashboard's existing single_stat.vue visualization.
• Validation: the query must have exactly 1 metric and 0 dimensions; otherwise the presenter emits an error (same pattern as the chart presenters).
• Value formatting reuses the chart unit registry (formatterFor), so counts (1,234), rates (73.5%), and durations render consistently across visualizations. A metric with no registered unit renders its raw value; the fix for a new metric is a one-line registry entry that benefits charts and stats alike.

Bonus:

• I've noticed we are still showing the item count in the CRUD container with mode analytics ( pretty sure we removed it at some point, but we must have accidentally revert it ), so hiding it again here.

Intentionally out of scope (per the issue):

• Trend % change indicator — tracked in #592912 (moved) (showTrends). It maps onto GlSingleStat's meta-text/meta-icon when it lands.
• Title icon — owned by the container alongside the title.

Screenshots or screen recordings

Query	Output
mode: analytics query: type = codesuggestion and group = "gitlab-duo" metrics: totalCount display: stat
mode: analytics query: type = codesuggestion and group = "gitlab-duo" metrics: totalCount dimensions: user display: stat
query: type = issue and group = "gitlab-duo" display: stat

How to set up and validate locally

Render a GLQL block with display: stat and a single metric, e.g.:

```glql
mode: analytics
query: type = codesuggestion and group = "gitlab-duo"
metrics: totalCount
display: stat
```

References

• Resolves glql#106 (closed)