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-tauri/Cargo.lock

@@ -4236,10 +4236,12 @@ version = "0.1.0"
 dependencies = [
  "base64 0.22.1",
  "chrono",
+ "percent-encoding",
  "rusqlite",
  "rusqlite_migration",
  "serde",
  "serde_json",
+ "sha2",
  "tauri",
  "tauri-build",
  "tauri-plugin-dialog",