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/electron/main.cjs

@@ -722,7 +722,7 @@ function broadcastBootstrapEvent(ev) {
       error: ev.error ?? null
     }
   } else if (ev.type === 'log') {
-    bootstrapState.log.push({ ts: Date.now(), stage: ev.stage || null, line: ev.line })
+    bootstrapState.log.push({ ts: Date.now(), stage: ev.stage || null, line: ev.line, stream: ev.stream || 'stdout' })
     if (bootstrapState.log.length > BOOTSTRAP_LOG_RING_MAX) {
       bootstrapState.log.splice(0, bootstrapState.log.length - BOOTSTRAP_LOG_RING_MAX)
     }