fix(import): guard unparseable CSV dates and never wedge the button (L7)

A non-empty invalid transaction_date ('N/A') passed presence-only validation,
then new Date(str).toISOString() threw mid-import: nothing imported, no toast,
and setImporting(false) never ran, sticking the button on 'Importing...'.
Extracted parsePaymentRow to lib/import-logic.ts with a date guard, and wrapped
runImport so the importing flag always clears.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

__tests__/import-logic.test.ts

@@ -0,0 +1,44 @@
+import { describe, it, expect } from 'vitest'
+import { parsePaymentRow } from '@/lib/import-logic'
+
+describe('parsePaymentRow', () => {
+  it('parses a well-formed row into a normalized payment', () => {
+    const r = parsePaymentRow(
+      { amount: '$1,200.50', from_identifier: 'Ada', from_note: 'dues', transaction_date: '2026-01-15', platform: 'venmo' },
+      2,
+    )
+    expect(r.ok).toBe(true)
+    if (r.ok) {
+      expect(r.value.amount).toBe(1200.5)
+      expect(r.value.platform).toBe('venmo')
+      expect(r.value.from_identifier).toBe('Ada')
+      expect(r.value.transaction_date).toBe(new Date('2026-01-15').toISOString())
+    }
+  })
+
+  it('defaults platform to csv when unmapped', () => {
+    const r = parsePaymentRow({ amount: '10', from_identifier: 'Bo', transaction_date: '2026-02-01' }, 3)
+    expect(r.ok).toBe(true)
+    if (r.ok) expect(r.value.platform).toBe('csv')
+  })
+
+  // The bug: a non-empty but unparseable date passed presence validation, then
+  // new Date(str).toISOString() threw RangeError mid-map, wedging the import.
+  it('reports an error (never throws) for a non-empty unparseable date', () => {
+    expect(() =>
+      parsePaymentRow({ amount: '10', from_identifier: 'Cy', transaction_date: 'N/A' }, 4),
+    ).not.toThrow()
+    const r = parsePaymentRow({ amount: '10', from_identifier: 'Cy', transaction_date: 'N/A' }, 4)
+    expect(r.ok).toBe(false)
+    if (!r.ok) expect(r.error).toBe('Row 4: unparseable date "N/A"')
+  })
+
+  it('reports missing required fields', () => {
+    expect(parsePaymentRow({ amount: 'nope', from_identifier: 'Di', transaction_date: '2026-01-01' }, 5)).toEqual({
+      ok: false,
+      error: 'Row 5: missing required fields',
+    })
+    expect(parsePaymentRow({ amount: '10', from_identifier: '', transaction_date: '2026-01-01' }, 6).ok).toBe(false)
+    expect(parsePaymentRow({ amount: '10', from_identifier: 'Di', transaction_date: '' }, 7).ok).toBe(false)
+  })
+})