What?

Added import format documentation and expanded export options in the Import/Export dialogs, along with header UI polish.

• Import dialog: Added an info icon next to the description that opens a popover explaining the expected CSV format (required header row with Time, Channel1–4 columns and RFC 3339 timestamps).
• Export dialog: Added a "Last duration / All data" toggle. "All data" omits start_time/end_time from the request, letting the backend export the full session range. Added a warning message when no active session is loaded. Fixed dialog height to be static across both modes.
• Header: Fixed vertical logo centering and tightened icon-to-text spacing on Export/Import buttons to match the Help chevron.

How?

• The info popover in the import dialog uses the existing Radix UI Popover component with an InfoCircledIcon trigger inline with the DialogDescription.
• The export mode toggle is a plain segmented button that sets exportMode state ('range' | 'all'). When 'all' is selected, the options object passed to exportEEGData is left empty - the backend already treats missing start_time/end_time as "from earliest to now".
• The changing section of the export dialog is wrapped in a fixed h-20 container to prevent layout shift between modes.
• Header logo centering was fixed by splitting p-2 into px-2 py-3. Button icon spacing was changed from space-x-1 to gap-1 to override shadcn Button's internal gap behaviour.