NEARBY: Add back the live user count

## **⚠️ Problem:**
NEARBY tab title lost its user count during the Social panel redesign — code comment confirms it's now static ("NEARBY", no count), unlike FRIENDS/REQUESTS which keep "(n)" labels. Users can't tell scene population at a glance without opening the tab.

## **🏁 Scope:**
Restore a live count on the NEARBY tab (`Nearby (n)`), matching the FRIENDS/REQUESTS "(n)" pattern already in `friends_panel.gd`. Driven by `nearby_list.list_size`, updated on `size_changed` like the other tabs.

## **📝 Deliverables:**
- [ ] NEARBY tab title shows `Nearby (n)`, n = current nearby player count
- [ ] Count updates live on `nearby_list.size_changed`, same trigger as FRIENDS/REQUESTS counts
- [ ] Count hidden/omitted at 0, consistent with empty-state handling already in place

## **🔗 References:**
- [decentraland/godot-explorer#2792](https://github.com/decentraland/godot-explorer/pull/2792) — Social section redesign PR that shipped the NEARBY tab as static title, no count (regression source)
- [decentraland/godot-explorer#2786](https://github.com/decentraland/godot-explorer/issues/2786) — Social Section Redesign spec issue; Figma scope never called out dropping the count