Redesign the Aperture dashboard workspace

[mayvqt]

Summary

• pin the desktop navigation rail and give dashboard content more breathing room
• compact the administrator login while preserving Aperture's charcoal and coral theme
• add a polished Remember me control with a tested seven-day session
• keep the responsive mobile layout free of horizontal overflow

Validation

• go test ./internal/httpserver
• go test ./...
• go vet ./...
• git diff --check
• desktop and mobile browser visual QA