feat: Preview feature

Resolves part of #583 (closed)

• Adds preview feature (usePreviewMode hook, PreviewModeContext, and PreviewAutocomplete component)
• Adds "standby" page for non-member organization users with elevated access and/or admin
• Extracts layouts from pages into reusable components (FullLayout and BackgroundBoxLayout)
• Extracts account menu into a component AccountMenu