Add issue/cancel lifecycle, GST derivation and integer-paise totals

issue_invoice allocates the number, freezes vendor and bank details and
stores server-computed totals in one transaction. Issued invoices are
cancelled, not deleted. Tax heads derive from supplier state and place of
supply (a disagreeing choice is rejected); unregistered suppliers issue a
plain Invoice. Adds GSTIN checksum validation, canonical India Compliance
state list, server-side drafts, series validation, relative and
magic-byte-checked asset paths, and GST settings. PDF re-exports use the
frozen vendor snapshot.

src/lib/pdf.tsx

@@ -17,12 +17,24 @@ function blobToBase64(blob: Blob): Promise<string> {
   });
 }
 
+function vendorFromSnapshot(snapshot: string | undefined, live: Settings): Settings {
+  if (!snapshot) return live;
+  try {
+    return { ...live, ...(JSON.parse(snapshot) as Partial<Settings>) };
+  } catch {
+    return live;
+  }
+}
+
 export async function buildPdfProps(
   invoice: Invoice,
   settings: Settings,
   bank: BankAccount | null,
 ): Promise<InvoicePdfProps> {
-  const logoDataUri = settings.logoPath ? await api.readAssetDataUri(settings.logoPath) : null;
+  // Re-exports use the vendor details frozen at issue; live settings are only a fallback for
+  // legacy rows without a snapshot.
+  const vendor = vendorFromSnapshot(invoice.vendorSnapshot, settings);
+  const logoDataUri = vendor.logoPath ? await api.readAssetDataUri(vendor.logoPath) : null;
   const signatureDataUri = invoice.signaturePath
     ? await api.readAssetDataUri(invoice.signaturePath)
     : null;
@@ -37,7 +49,7 @@ export async function buildPdfProps(
   }
 
   return {
-    vendor: settings,
+    vendor,
     bank: resolvedBank,
     logoDataUri,
     signatureDataUri,