feat: view and edit a transaction's note from the list

Transactions can carry a `notes` field (set via the API — e.g. the
currency-conversion metadata on this user's imported historical data), but
there was no way to see it in the UI at all.

TransactionsTable now shows a note icon next to edit/delete, only when a
transaction actually has one, opening a dialog with the text. The edit
form's `notes` field was already wired end to end (state, payload) but had
no input in the JSX — added it, since a view-only note with no way to edit
or clear it would be a dead end.

Also fixed e2e/run.sh's readiness check, found while running the full
suite for this change: it polled `/` (nginx, answers almost immediately)
instead of `/api/health`, so the first test could start before the backend
finished migrating/seeding and fail trying to parse nginx's HTML error
page as JSON.

frontend/src/components/transactions/TransactionFormModal.tsx

@@ -233,6 +233,15 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
           />
         </div>
 
+        <div>
+          <Label htmlFor="notes">{t("transactions.form.notesLabel")}</Label>
+          <Input
+            id="notes"
+            value={form.notes}
+            onChange={(event) => setForm((prev) => ({ ...prev, notes: event.target.value }))}
+          />
+        </div>
+
         {error && <p className="text-sm text-danger">{error}</p>}
 
         <div className="flex justify-end gap-2 pt-2">