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/global.d.ts

@@ -222,7 +222,7 @@ export interface DesktopBootstrapState {
   manifest: { type: 'manifest'; stages: DesktopBootstrapStageDescriptor[]; protocolVersion: number | null } | null
   stages: Record<string, DesktopBootstrapStageResult>
   error: string | null
-  log: Array<{ ts: number; stage: string | null; line: string }>
+  log: Array<{ ts: number; stage: string | null; line: string; stream?: 'stdout' | 'stderr' }>
   startedAt: number | null
   completedAt: number | null
   unsupportedPlatform: DesktopBootstrapUnsupportedPlatform | null
@@ -238,7 +238,7 @@ export type DesktopBootstrapEvent =
       json?: DesktopBootstrapStageResult['json']
       error?: string | null
     }
-  | { type: 'log'; stage?: string | null; line: string }
+  | { type: 'log'; stage?: string | null; line: string; stream?: 'stdout' | 'stderr' }
   | { type: 'complete'; marker: Record<string, unknown> }
   | { type: 'failed'; stage?: string | null; error: string }
   | {