ui: small-tablet (640–900px) viewports lose the region counter

[comnam90]

Spun out of code review on #103 (item #10).

Context

The new `.status` HUD strip hides below 900px:

```css
@media (max-width: 900px) {
.status { display: none; }
}
```

The previous `#regionCount` block was visible from the `sm:` breakpoint (≥640px). Result: small tablets in portrait (640–900px wide) no longer see how many regions are currently visible vs. total.

Proposed fix

Either:

• Lower the breakpoint to `max-width: 640px` so tablets keep the counter.
• Or keep the strip hidden but surface the count in the controls row on mid-range viewports.
• Or decide the loss is acceptable and document it in ADR-004.

Acceptance

• Decision documented (either restored or explicitly accepted in ADR-004)
• If restored: counter visible on 640–900px viewports