Fix/resolve 4 invoice component issues (#588, #587, #586, #585)

[lolandriley-wq]

Summary

This PR addresses 4 related invoice component issues:

• InvoiceListSentinel does not disconnect IntersectionObserver on unmount #588: InvoiceListSentinel observer cleanup (verification)
• Disable InvoiceExportButton and show spinner while export is in progress #587: Disable InvoiceExportButton and show spinner during export
• Extract line-item rows from InvoiceSection into a standalone LineItemRow component #586: Extract line-item rows into standalone LineItemRow component
• Add smooth expand/collapse animation to InvoiceTimeline entries #585: Add smooth expand/collapse animation to InvoiceTimeline entries

Changes

Issue #587 - InvoiceExportButton

• Added spinner icon that displays while export is in progress
• Button is disabled immediately after first click
• Shows "Generating..." text alongside spinner
• Returns to normal state after export completes
• Error toast shown if export fails

Issue #586 - LineItemRow Component

• Created new LineItemRow.tsx component in /src/components
• Extracted inline recipient row JSX from RecipientForm
• Fully typed with TypeScript
• Supports edit, delete, and amount suggestion functionality
• Updated RecipientForm to use the new LineItemRow component

Issue #585 - InvoiceTimeline Animation

• Added expand/collapse functionality to timeline entries with details
• Smooth CSS height transitions (300ms duration)
• "Show details" / "Hide details" toggle button
• Animation respects prefers-reduced-motion media query
• Details include actor address and transaction hash

Tests

• Added comprehensive test suite for all fixed components
• InvoiceListSentinel: Observer cleanup verification
• InvoiceExportButton: Button state and spinner display
• LineItemRow: Component rendering and user interactions
• InvoiceTimeline: Expand/collapse and animation behavior

Closes #588
Closes #587
Closes #586
Closes #585

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