docs: document download peer count chip

Update the launcher design reference so active downloads show how many LAN
peers are currently seeding the transfer. The reference now places the peer
chip between speed and ETA, describes the singular/plural copy, and records
how the ETA and peer count collapse in narrow modal layouts.

Test Plan:
- git diff --cached --check

design/design_reference/styles.css

@@ -784,6 +784,12 @@
 }
 .dl-lg-secondary .dl-bytes .dl-of { color: var(--t-2); font-weight: 500; }
 .dl-lg-secondary .dl-speed { color: var(--t-1); font-weight: 600; white-space: nowrap; }
+.dl-lg-secondary .dl-peers {
+  display: inline-flex; align-items: center; gap: 5px;
+  color: var(--t-2); white-space: nowrap;
+}
+.dl-lg-secondary .dl-peers strong { color: var(--t-1); font-weight: 600; font-variant-numeric: tabular-nums; }
+.dl-lg-secondary .dl-peers svg { opacity: 0.7; }
 .dl-lg-secondary .dl-eta { white-space: nowrap; min-width: 0; overflow: hidden; text-overflow: ellipsis; }
 .dl-sep { opacity: 0.45; }
 
@@ -792,6 +798,11 @@
   .dl-lg-secondary .dl-eta,
   .dl-lg-secondary .dl-sep-eta { display: none; }
 }
+/* Even narrower: drop peers too, keep size + speed */
+@container (max-width: 300px) {
+  .dl-lg-secondary .dl-peers,
+  .dl-lg-secondary .dl-sep-peers { display: none; }
+}
 
 .dl-lg-pct {
   grid-area: pct;