feat: six UX improvements (drop zone, cancel, preview, re-export, no blob regen, webhook badge)

[davidamunga]

Summary

• Drop zone fully clickable — clicking anywhere on the upload area opens the file picker, not just the "Browse" button.
• Cancel during processing — a Cancel button appears while PDF extraction runs; clicking it kills the Java process (cancel_pdf_extraction) and returns to the idle state.
• Transaction preview before export — a collapsible table (first 10 rows) appears once parsing is complete so users can verify data before downloading.
• Re-export without Start Again — the Export button is always visible after parsing; changing format or options and clicking Export again works immediately without resetting.
• XLSX blob only on Export click — removed eager createDownloadLink calls from handleFormatChange and handleOptionsChange; the export buffer is now built only when the user actually clicks Export.
• Webhook result badge — after sending, the panel stays open and a colour-coded ✓/✗ badge appears on the section header so the result is visible even if the user collapses the panel.

Files changed

File	Changes
src/components/file-uploader.tsx	onClick + keyboard handler on drop zone div
src/App.tsx	cancel ref + handleCancel, preview state, stripped eager blob, always-visible Export button
src/components/export-options.tsx	webhook badge + auto-open panel after send
.changeset/feat-ux-improvements.md	minor changeset

Test plan

• Click empty drop zone area (not the button) → file picker opens
• Drop a PDF → Cancel during processing → returns to idle with no error shown
• Parse a statement → expand Preview panel → verify first 10 rows
• Parse → Export → change format → Export again without Start Again
• Toggle sheet options rapidly → observe no lag (no blob being generated)
• Send to webhook → collapse panel → badge visible on header

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