fix: popups no longer close on backdrop click, only via explicit close/cancel

Clicking the dimmed area outside a modal (+Task, +Invoice, expense,
review, etc.) accidentally dismissed it and lost in-progress input.
Every popup already has an explicit Cancel/Close/✕ button, so the
backdrop click handler is removed; also drops the now-dead
onClose/blockClose prop plumbing that only existed to support it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/pages/external/ExternalMyInvoices.jsx

@@ -428,7 +428,7 @@ export default function MyInvoices() {
       </div>
 
       {showInvoiceForm && (
-        <div style={popupOverlayStyle} onClick={() => setShowInvoiceForm(false)}>
+        <div style={popupOverlayStyle}>
           <div
             style={{ ...popupSurfaceStyle, width: '80vw', height: '80vh', display: 'flex', flexDirection: 'column', gap: 0 }}
             onClick={(e) => e.stopPropagation()}
@@ -469,8 +469,6 @@ export default function MyInvoices() {
             title={inv?.invoice_number || 'Subcontractor Invoice'}
             subtitle={`${currentUser?.name || 'Subcontractor'}${currentUser?.email ? ` · ${currentUser.email}` : ''}`}
             headerRight={inv ? <StatusBadge status={STATUS_BADGE[inv.status] || 'not_started'} label={invoiceStatus} /> : null}
-            onClose={() => { if (!busy) { setViewingInvoice(null); setDetailError(''); } }}
-            blockClose={busy}
             loading={detailLoading && !inv}
             metaContent={inv ? <>
               <div><div style={F}>Created</div><div style={{ fontSize: 13, color: 'var(--text-primary)' }}>{inv.created_at ? new Date(inv.created_at).toLocaleDateString('en-US', { month: 'short', day: 'numeric', year: 'numeric' }) : '—'}</div></div>