Add ERPNext push logic for invoices and payments (Phase F2)

- ErpnextSink implements InvoiceSink. Pushing an issued invoice ensures the Customer and Address (read-back of the returned name, local PIN/state/GSTIN checks), creates the Sales Invoice, optionally submits it in a second call, and attaches the archived PDF as a private file once.
- Idempotent: a mirrored-name 409 is compared by grand total (same total records as synced, different total is a conflict and never overwrites); series mode looks up an existing document by its remarks before re-posting; an identical re-push sends nothing.
- Create and submit are separate calls so a draft survives a failed submit. A failed push is recorded in erpnext_sync with a readable message. Bulk push never aborts on one failing row.
- Payment push requires a submitted parent, maps TDS to Payment Entry deductions and is idempotent through the stored entry name.
- Migration M11 rebuilds erpnext_sync to allow a conflict status and adds attachment_sha256.
- Commands for single and bulk push, payment push, sync status and the open-in-ERPNext URL, with typed wrappers. No UI yet.

Tested against a mock server only. The Payment Entry deductions fields, the series-mode submit route and the payment amount semantics need a live ERPNext check.

src/lib/erpnext.ts

@@ -88,3 +88,67 @@ export interface ErpnextOptions {
   /** Lists that failed to load; the others are still filled. */
   errors: { list: string; message: string }[];
 }
+
+export type ErpnextErrorKind =
+  | "auth"
+  | "permission"
+  | "not_found"
+  | "duplicate"
+  | "validation"
+  | "rate_limit"
+  | "server"
+  | "timeout"
+  | "network"
+  | "config"
+  | "protocol"
+  | "conflict"
+  | "precondition"
+  | "other";
+
+/** `refused`: nothing was sent and nothing was recorded (a draft or cancelled invoice). */
+export type ErpnextPushStatus = "synced" | "error" | "conflict" | "refused";
+
+export interface ErpnextPushResult {
+  invoiceId: number;
+  number: string;
+  ok: boolean;
+  status: ErpnextPushStatus;
+  remoteName: string;
+  /** 0 draft, 1 submitted. */
+  remoteDocstatus: number;
+  /** Created by this call, not found already there. */
+  created: boolean;
+  /** Already synced with the same payload; nothing was done. */
+  noOp: boolean;
+  /** The archived PDF is attached on the remote document. */
+  attached: boolean;
+  error: string | null;
+  errorKind: ErpnextErrorKind | null;
+  /** Non-fatal notes, e.g. sent without the PDF because the invoice is not archived. */
+  warnings: string[];
+}
+
+export interface ErpnextPaymentPushResult {
+  paymentId: number;
+  invoiceId: number;
+  ok: boolean;
+  entryName: string | null;
+  /** Sent earlier; nothing was posted. */
+  alreadySynced: boolean;
+  error: string | null;
+  errorKind: ErpnextErrorKind | null;
+}
+
+/** `none` until something was pushed. */
+export type ErpnextSyncState = "none" | "pending" | "synced" | "error" | "conflict";
+
+export interface ErpnextSyncStatus {
+  invoiceId: number;
+  status: ErpnextSyncState;
+  remoteName: string;
+  /** 0 draft, 1 submitted. */
+  remoteDocstatus: number;
+  lastError: string;
+  syncedAt: string | null;
+  attached: boolean;
+}