feat: Export List (Issue #5)

[merlin1de]

Summary

• New Export List button next to "Share" — opens a modal with CSV preview, download and copy to clipboard
• Selectable columns: filename (mandatory), stars, color, pick/reject
• Guests can use export if allow_export is enabled on the share (default: off)
• Share modal and share list: toggle for export permission
• Fixes: ESC closes modal, button focus after ESC, filter operator after reset (≥ instead of =), version 1.2.1

Do not merge

• Do not merge into master until explicitly approved
• Do not publish a new app version

Closes

Closes #5