Add versioned DB migrations, startup error dialog and native basics

rusqlite_migration with a VACUUM INTO backup before upgrading; M2 adds GST
registration, snapshot, cancellation and draft columns and backfills
snapshots for existing invoices. Startup failures now show a native dialog
instead of exiting silently. Adds single-instance and window-state plugins,
minimum window 960x600, embedBootstrapper, DMABUF workaround on Linux and
xbps dependency updates.

src-tauri/src/models.rs

@@ -19,6 +19,10 @@ pub struct Settings {
     pub currency: String,
     pub onboarded: bool,
     pub theme: String,
+    pub gst_registration: String,
+    pub default_hsn_sac: String,
+    pub signatory_name: String,
+    pub signatory_designation: String,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]