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/App.tsx

@@ -88,9 +88,19 @@ export default function App() {
   return (
     <Theme theme={theme}>
       <AppShell view={view} onNavigate={setView} theme={settings.theme} onToggleTheme={toggleTheme}>
-        {view === "new" && <NewInvoice settings={settings} onSettingsChange={refreshSettings} />}
-        {view === "invoices" && <InvoiceHistory settings={settings} />}
-        {view === "series" && <SeriesSettings />}
+        {/* Main views stay mounted so a half-typed invoice survives switching tabs. */}
+        <NewInvoice
+          settings={settings}
+          onSettingsChange={refreshSettings}
+          active={view === "new"}
+          onActivate={() => setView("new")}
+        />
+        <div hidden={view !== "invoices"}>
+          <InvoiceHistory settings={settings} active={view === "invoices"} />
+        </div>
+        <div hidden={view !== "series"}>
+          <SeriesSettings active={view === "series"} />
+        </div>
         {view === "settings" && <AppSettings settings={settings} onSaved={refreshSettings} />}
       </AppShell>
     </Theme>