Replace the HTML preview with a worker-rendered PDF preview

Rendering moves into a module Web Worker behind a latest-wins RenderClient
with a watchdog and a handshake-based main-thread fallback. The preview
shows the real PDF through pdf.js at 2x with zoom, page indicator, audit
issues popover and a focus modal. A4/Letter and margin controls are frozen
into each invoice at issue and can be saved as the default (migration M3).
react-pdf and pdf.js are lazy-loaded; the main chunk shrinks from 1.8 MB
to 0.5 MB. The HTML twin and its styles are removed.

src/lib/api.ts

@@ -28,7 +28,7 @@ export const api = {
   deleteClient: (id: number) => invoke<void>("delete_client", { id }),
 
   peekNextInvoiceNumber: () => invoke<string>("peek_next_invoice_number"),
-  issueInvoice: (input: InvoiceInput, renderPrefs: Record<string, unknown>) =>
+  issueInvoice: (input: InvoiceInput, renderPrefs: object) =>
     invoke<Invoice>("issue_invoice", { input, renderPrefs }),
   cancelInvoice: (id: number, reason: string) =>
     invoke<Invoice>("cancel_invoice", { id, reason }),