Add PDF archive store, raw-body IPC and export file helpers

The searchable PDF can be archived content-addressed under the local data
dir with its sha256 and layout fingerprint; a different PDF for an archived
invoice is refused and reads verify the hash. Export writes go through a
raw-body command that remembers the last folder; reveal-in-folder and open
fall back to xdg-open / explorer. Adds a Windows-safe export file name
builder. Migration M4 adds the archive columns and last export dir.

src/lib/pdf.tsx

@@ -10,13 +10,6 @@ import { imageSizeFromDataUri } from "./imageSize";
 import { getRenderClient, nextRenderJobId } from "./renderClient";
 import type { BankAccount, Invoice, Settings } from "./types";
 
-function bytesToBase64(bytes: Uint8Array): string {
-  let bin = "";
-  const step = 0x8000;
-  for (let i = 0; i < bytes.length; i += step) bin += String.fromCharCode(...bytes.subarray(i, i + step));
-  return btoa(bin);
-}
-
 /** A logo with no readable size is fitted as if it filled Classic's 170 x 54 box. */
 const UNKNOWN_LOGO_SIZE = { width: 170, height: 54 };
 
@@ -82,7 +75,7 @@ export async function savePdfBytes(bytes: Uint8Array, fileName: string): Promise
     filters: [{ name: "PDF document", extensions: ["pdf"] }],
   });
   if (!path) return null;
-  await api.saveBinaryFile(path, bytesToBase64(bytes));
+  await api.writeExportFile(path, bytes);
   return path;
 }