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/types.ts

@@ -25,6 +25,8 @@ export interface Settings {
   signatoryDesignation: string;
   /** The user's default page setup (RenderPrefsV1 JSON); empty means the built-in defaults. */
   renderPrefs: string;
+  /** Folder of the last export; kept by the backend (saving settings does not change it). */
+  lastExportDir?: string;
 }
 
 export interface BankAccount {
@@ -136,6 +138,9 @@ export interface Invoice {
   archivedPdfSha256: string | null;
   /** RenderPrefsV1 JSON frozen at issue; empty when none was stored. */
   renderPrefs: string;
+  /** Layout fingerprint of the archived PDF; null when nothing is archived. */
+  archivedFingerprint?: string | null;
+  archivedAt?: string | null;
   items: InvoiceItem[];
 }