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/src/commands/settings.rs

@@ -6,7 +6,7 @@ use tauri::State;
 pub(crate) const SETTINGS_COLS: &str = "vendor_name, vendor_address, vendor_email, vendor_phone, vendor_pan,
     vendor_gstin, vendor_state_code, logo_path, signature_path, default_bank_id, default_tax_rate,
     default_tax_type, payment_terms_days, currency, onboarded, theme, gst_registration, default_hsn_sac, signatory_name,
-    signatory_designation, render_prefs";
+    signatory_designation, render_prefs, last_export_dir";
 
 pub(crate) fn map_settings(row: &Row) -> rusqlite::Result<Settings> {
     Ok(Settings {
@@ -31,6 +31,7 @@ pub(crate) fn map_settings(row: &Row) -> rusqlite::Result<Settings> {
         signatory_name: row.get(18)?,
         signatory_designation: row.get(19)?,
         render_prefs: row.get(20)?,
+        last_export_dir: row.get(21)?,
     })
 }