Add issue/cancel lifecycle, GST derivation and integer-paise totals

issue_invoice allocates the number, freezes vendor and bank details and
stores server-computed totals in one transaction. Issued invoices are
cancelled, not deleted. Tax heads derive from supplier state and place of
supply (a disagreeing choice is rejected); unregistered suppliers issue a
plain Invoice. Adds GSTIN checksum validation, canonical India Compliance
state list, server-side drafts, series validation, relative and
magic-byte-checked asset paths, and GST settings. PDF re-exports use the
frozen vendor snapshot.

src-tauri/src/lib.rs

@@ -1,5 +1,6 @@
 mod commands;
 mod db;
+mod gst;
 mod models;
 
 use rusqlite::Connection;
@@ -46,6 +47,10 @@ fn init_state(app: &tauri::App) -> Result<AppState, Box<dyn std::error::Error>>
             return Err(e.into());
         }
     };
+    if let Err(e) = db::relativize_asset_paths(&conn, &data_dir) {
+        show_startup_error(app, &format!("Could not update stored image paths: {e}"), Some(&data_dir));
+        return Err(e.into());
+    }
     Ok(AppState {
         db: Mutex::new(conn),
         data_dir,
@@ -96,10 +101,14 @@ pub fn run() {
             commands::clients::save_client,
             commands::clients::delete_client,
             commands::invoice::peek_next_invoice_number,
-            commands::invoice::create_invoice,
+            commands::invoice::issue_invoice,
+            commands::invoice::cancel_invoice,
             commands::invoice::list_invoices,
             commands::invoice::get_invoice,
-            commands::invoice::delete_invoice,
+            commands::invoice::save_draft,
+            commands::invoice::list_drafts,
+            commands::invoice::get_draft,
+            commands::invoice::delete_draft,
             commands::assets::import_asset,
             commands::assets::save_asset_bytes,
             commands::assets::read_asset_data_uri,