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/bootstrap-installer/src/routes/progress.tsx

@@ -115,9 +115,7 @@ export default function ProgressScreen({ bootstrap }: ProgressProps) {
                   key={idx}
                   className={clsx(
                     'whitespace-pre-wrap',
-                    entry.line.startsWith('stderr:')
-                      ? 'text-destructive'
-                      : 'text-foreground/70'
+                    entry.stream === 'stderr' ? 'text-foreground/45' : 'text-foreground/70'
                   )}
                 >
                   {entry.line}