fix: resolve 4 critical component issues

[lolandriley-wq]

Summary

This PR fixes 4 critical issues with component functionality and UX improvements.

Changes

• InvoiceListSentinel does not disconnect IntersectionObserver on unmount Stellar-split/split-app#588: InvoiceListSentinel - Ensure IntersectionObserver disconnects on unmount to prevent stale callbacks and redundant API calls
• Disable InvoiceExportButton and show spinner while export is in progress Stellar-split/split-app#587: InvoiceExportButton - Disable button during export and show spinner icon while generating PDF, with error toast on failure
• Extract line-item rows from InvoiceSection into a standalone LineItemRow component Stellar-split/split-app#586: Extract LineItemRow component - New focused component for rendering line items with support for edit, delete, and drag operations
• Add smooth expand/collapse animation to InvoiceTimeline entries Stellar-split/split-app#585: InvoiceTimeline - Add smooth height transition animation (300ms) for expand/collapse with prefers-reduced-motion support

Testing

All changes maintain backward compatibility and existing tests continue to pass.

Closes Stellar-split#588
Closes Stellar-split#587
Closes Stellar-split#586
Closes Stellar-split#585