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/types.ts

@@ -23,6 +23,8 @@ export interface Settings {
   defaultHsnSac: string;
   signatoryName: string;
   signatoryDesignation: string;
+  /** The user's default page setup (RenderPrefsV1 JSON); empty means the built-in defaults. */
+  renderPrefs: string;
 }
 
 export interface BankAccount {
@@ -132,6 +134,8 @@ export interface Invoice {
   cancelledAt: string | null;
   cancelReason: string;
   archivedPdfSha256: string | null;
+  /** RenderPrefsV1 JSON frozen at issue; empty when none was stored. */
+  renderPrefs: string;
   items: InvoiceItem[];
 }