ExportModal: disabled '(empty)' checkboxes are visual noise

[stultus]

Area: src/lib/components/ExportModal.svelte
Severity: Low

Problem

When Synopsis / Treatment / Narrative are empty, their checkboxes still render
(just disabled, with "(empty)" appended). This clutters the list and draws
attention to things the user can't actually export.

Suggested fix

Options:

• Hide empty sections from the list entirely.
• Group them under a collapsed "Unavailable" block.
• Keep them visible but render much more subtly (smaller, no "(empty)" suffix).