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-tauri/src/commands/payments.rs

@@ -30,6 +30,9 @@ pub struct Payment {
     pub reference: String,
     pub notes: String,
     pub created_at: String,
+    /// Name of the Payment Entry on ERPNext once this payment was sent; `None` until then.
+    #[serde(default)]
+    pub erpnext_payment_entry: Option<String>,
 }
 
 #[derive(Debug, Clone, Deserialize)]
@@ -127,10 +130,11 @@ fn map_payment(r: &rusqlite::Row) -> rusqlite::Result<Payment> {
         reference: r.get(6)?,
         notes: r.get(7)?,
         created_at: r.get(8)?,
+        erpnext_payment_entry: r.get::<_, Option<String>>(9)?.filter(|e| !e.trim().is_empty()),
     })
 }
 
-const COLS: &str = "id, invoice_id, paid_on, amount_paise, tds_paise, mode, reference, notes, created_at";
+const COLS: &str = "id, invoice_id, paid_on, amount_paise, tds_paise, mode, reference, notes, created_at, erpnext_payment_entry";
 
 pub fn record_payment_impl(conn: &mut Connection, input: PaymentInput) -> Result<Payment, String> {
     let db = |e: rusqlite::Error| e.to_string();