Add the ERPNext settings UI, History and detail controls, and voiced.invoice.v1 export (Phase F3)

- Settings gets an Integrations tab: connection fields with a write-only API secret, a CA PEM option, Test connection with versions and warnings, live mapping lists loaded after a test, naming mode, toggles and its own Save.
- Pushing for a GST-registered vendor is refused until a connection test result is stored. The UI shows the reason and disables Send.
- History shows an ERPNext status column from one bulk call, a row Send and Open in ERPNext, and a bulk Send selected with a draft-or-submit confirm step and per-row results that never abort the batch. The detail view gets an ERPNext panel and per-payment send. All of it is hidden or disabled when the integration is not configured.
- Optional auto-push after issue runs in the background and never blocks or fails the issue.
- Generic export: a deterministic voiced.invoice.v1 JSON (and CSV line items) for selected invoices, documented in docs/voiced-invoice-v1.md with a golden test. Money is integer paise plus a decimal string.

The new screens have not been run in a webview yet. The ERPNext Payment Entry deduction fields still need a live check.

src/lib/historyExport.ts

@@ -154,7 +154,7 @@ export interface HistoryExportDeps {
   now(): Date;
 }
 
-function joinPath(dir: string, name: string): string {
+export function joinPath(dir: string, name: string): string {
   if (!dir) return name;
   if (/[\\/]$/.test(dir)) return `${dir}${name}`;
   return `${dir}${dir.includes("\\") && !dir.includes("/") ? "\\" : "/"}${name}`;