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.toml

@@ -26,6 +26,8 @@ chrono = { version = "0.4", features = ["serde"] }
 uuid = { version = "1", features = ["v4"] }
 thiserror = "2"
 base64 = "0.22"
+sha2 = "0.10"
+percent-encoding = "2"
 
 # Desktop-only plugins (not available on mobile targets).
 [target.'cfg(not(any(target_os = "android", target_os = "ios")))'.dependencies]