Add real-time build log streaming to dashboard

[Benbentwo]

Summary

Added real-time log streaming for ongoing builds in the dashboard. Build logs now stream to a collapsible terminal-style viewer underneath each editor instance card, providing live feedback during compilation with scroll-back history support.

Changes

Backend:

• Added RunBuildScriptPiped() to return piped command for build I/O capture
• Integrated LogCapture infrastructure into BuildOrchestrator for streaming build logs through ring buffer
• Implemented SSE endpoint /api/build/logs/stream with three event types: build_log_history (recent lines), build_log (live), build_log_end (completion)
• Added unit tests for log capture lifecycle and SSE behavior

Frontend:

• Added LogLineEvent type for typed log line events
• Created useBuildLogStream hook managing EventSource connection and cleanup
• Built BuildLogViewer component with terminal-style rendering, auto-scroll with scroll-back history, color-coded severity levels
• Extracted InstanceCard sub-component to enable proper React hook usage per instance

Logs only stream during active builds and display only for the instance with an ongoing build.