feat: show active download peer count

Render the active peer count already carried by download progress events in the
large download progress control. The peer chip appears between speed and ETA,
uses singular/plural copy, and hides after the ETA when the detail modal gets
very narrow.

This keeps the UI aligned with the design reference without changing backend
state ownership or download progress plumbing.

Test Plan:
- git diff --check
- git diff --cached --check
- just frontend-test
- just build

crates/lanspread-tauri-deno-ts/src/styles/launcher.css

@@ -1045,6 +1045,21 @@
   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 {
   min-width: 0;
   overflow: hidden;
@@ -1060,6 +1075,12 @@
     display: none;
   }
 }
+@container (max-width: 300px) {
+  .dl-lg-secondary .dl-peers,
+  .dl-lg-secondary .dl-sep-peers {
+    display: none;
+  }
+}
 .dl-lg-pct {
   grid-area: pct;
   color: var(--t-1);