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-tauri/src/commands/invoice.rs

@@ -17,7 +17,7 @@ const INVOICE_COLS: &str = "id, number, series_id, invoice_date, due_date, clien
     tax_type, tax_rate, cgst_amount, sgst_amount, igst_amount, total, amount_in_words,
     bank_account_id, bank_snapshot, signature_path, notes, status, created_at, updated_at,
     doc_type, reverse_charge, COALESCE(vendor_snapshot, ''), snapshot_origin, cancelled_at,
-    cancel_reason, archived_pdf_sha256";
+    cancel_reason, archived_pdf_sha256, COALESCE(render_prefs, '')";
 
 fn map_invoice(row: &Row) -> rusqlite::Result<Invoice> {
     Ok(Invoice {
@@ -55,6 +55,7 @@ fn map_invoice(row: &Row) -> rusqlite::Result<Invoice> {
         cancelled_at: row.get(31)?,
         cancel_reason: row.get(32)?,
         archived_pdf_sha256: row.get(33)?,
+        render_prefs: row.get(34)?,
         items: Vec::new(),
     })
 }
@@ -922,6 +923,7 @@ mod tests {
             .query_row("SELECT render_prefs FROM invoices", [], |r| r.get(0))
             .unwrap();
         assert_eq!(prefs, "{\"theme\":\"plain\"}");
+        assert_eq!(inv.render_prefs, prefs);
     }
 
     #[test]