fix(start): let the quick-access New button work on the Start screen

The button runs QNEW since #1635, and QNEW was missing from the
commands the Start screen allows, so it was refused until a drawing
was open.

Fixes #1728

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/commands/mod.rs

@@ -428,6 +428,7 @@ pub fn start_allowed(cmd: &str) -> bool {
         || matches!(
             cmd,
             "NEW"
+            | "QNEW"
             | "OPEN"
             | "EXIT"
             | "QUIT"