fix: sub billing only applies to work an external sub delivered

Report was flagging every unbilled delivered version as "Not Invoiced"
for sub billing, even when a team member delivered it — team work is
billed to the client directly and is never sub-invoiced. Now traces
each version's actual deliverer and shows N/A when it's a team member,
regardless of who delivered earlier versions on the same task.

Also: SubcontractorInvoiceForm no longer masks fetch errors as "No
completed tasks available to invoice" — the real error now shows in
the Completed Tasks panel.

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

src/components/SubcontractorInvoiceForm.jsx

@@ -347,6 +347,8 @@ export default function SubcontractorInvoiceForm({ embedded = false, onCancel, o
               </div>
               {loadingTasks ? (
                 <div style={{ fontSize: 12, color: 'var(--text-muted)' }}>Loading…</div>
+              ) : error ? (
+                <div style={{ fontSize: 12, color: 'var(--danger)' }}>{error}</div>
               ) : completedTasks.length === 0 ? (
                 <div style={{ fontSize: 12, color: 'var(--text-muted)' }}>No completed tasks available to invoice.</div>
               ) : (
@@ -442,6 +444,8 @@ export default function SubcontractorInvoiceForm({ embedded = false, onCancel, o
             </div>
             {loadingTasks ? (
               <p style={{ fontSize: 13, color: 'var(--text-muted)' }}>Loading...</p>
+            ) : error ? (
+              <p style={{ fontSize: 13, color: 'var(--danger)' }}>{error}</p>
             ) : completedTasks.length === 0 ? (
               <p style={{ fontSize: 13, color: 'var(--text-muted)' }}>No completed tasks available to invoice.</p>
             ) : (