Add invoice drafts, read-only issued state and keyboard shortcuts

Views stay mounted so switching tabs no longer loses a half-typed invoice.
The form autosaves a draft after 1 s, drafts can be resumed or deleted,
and after issuing the form is read-only with Export again, Duplicate and
New invoice actions. A failed export keeps the issued invoice and offers a
retry instead of issuing again. Adds Ctrl+N/S/Enter/L and a shortcut sheet;
F5 and Ctrl+R are blocked in production builds.

src/styles/carbon.scss

@@ -143,6 +143,11 @@ body {
   gap: 0.5rem;
 }
 
+// Views stay mounted while hidden so their state survives navigation.
+[hidden] {
+  display: none !important;
+}
+
 .voiced-page {
   padding: 2rem;
   max-width: 1600px;