Load the composer, previews, dialogs and other sidebars on demand

The main chunk carried everything the mail view might open: the file
preview and its Markdown renderer, the composer and its editor, the contact
editor, the filter and share dialogs, and the calendar, contacts and files
sidebars. Each is now loaded when first shown. The composer is also
fetched when the browser is idle after startup, so the first Compose does
not wait on the network.

Import the notification helpers statically where they already were: the
dynamic imports beside those static ones split nothing.

web/src/views/compose/ComposerDock.tsx

@@ -1,7 +1,20 @@
+import { lazy, Suspense } from "react";
 import { useCompose } from "@/store/compose";
-import { Composer } from "./Composer";
 import { useIsMobile } from "@/ui/misc";
 
+/*
+ * The composer -- the rich-text editor, the recipient and file pickers -- is
+ * loaded apart from the mail view, and fetched while the browser is idle
+ * after startup so the first Compose does not wait on the network.
+ */
+const loadComposer = () => import("./Composer");
+const Composer = lazy(() => loadComposer().then((m) => ({ default: m.Composer })));
+if (typeof window !== "undefined") {
+  const warm = () => void loadComposer().catch(() => {});
+  if ("requestIdleCallback" in window) window.requestIdleCallback(warm, { timeout: 5000 });
+  else setTimeout(warm, 2000);
+}
+
 export function ComposerDock() {
   const drafts = useCompose((s) => s.drafts);
   const activeKey = useCompose((s) => s.activeKey);
@@ -13,9 +26,11 @@ export function ComposerDock() {
   const hasMaximized = !isMobile && drafts.some((d) => d.maximized && !d.minimized);
   return (
     <div className={`composer-dock${hasMaximized ? " has-maximized" : ""}`}>
-      {visible.map((d) => (
-        <Composer key={d.key} draft={isMobile && d.key !== activeKey ? { ...d, minimized: true } : d} />
-      ))}
+      <Suspense fallback={null}>
+        {visible.map((d) => (
+          <Composer key={d.key} draft={isMobile && d.key !== activeKey ? { ...d, minimized: true } : d} />
+        ))}
+      </Suspense>
     </div>
   );
 }