fix(installer): stop mislabeling stdout-style progress as stderr

Both installers (Electron bootstrap-runner + Tauri) hardcoded a literal
`stderr: ` prefix onto every line that arrived on fd 2. Tools like
uv/pip/git/npm write normal progress to stderr by design, so routine
install output showed up tagged as "stderr" (and rendered red in the
Tauri progress UI), making a healthy install look like it was erroring.

Carry the stream as structured metadata (`stream: 'stdout' | 'stderr'`)
on the log event instead of mangling the line text. The UI now styles
stderr subtly (dimmed) rather than alarmingly, and the persistent
forensic logs keep their stdout/stderr distinction.

apps/desktop/src/components/desktop-install-overlay.tsx

@@ -177,7 +177,7 @@ function applyEvent(state: DesktopBootstrapState, ev: DesktopBootstrapEvent): De
     }
   }
   if (ev.type === 'log') {
-    const next = state.log.concat({ ts: Date.now(), stage: ev.stage ?? null, line: ev.line })
+    const next = state.log.concat({ ts: Date.now(), stage: ev.stage ?? null, line: ev.line, stream: ev.stream })
     while (next.length > 500) next.shift()
     return { ...state, log: next }
   }
@@ -431,7 +431,10 @@ export function DesktopInstallOverlay({ enabled = true }: DesktopInstallOverlayP
                 ) : (
                   <>
                     {state.log.map((entry, i) => (
-                      <div key={i} className="whitespace-pre-wrap break-words">
+                      <div
+                        key={i}
+                        className={cn('whitespace-pre-wrap break-words', entry.stream === 'stderr' && 'text-muted-foreground')}
+                      >
                         {entry.stage ? <span className="text-muted-foreground/70">[{entry.stage}] </span> : null}
                         <span>{entry.line}</span>
                       </div>