Let go of old message bodies and of exported files

Every message opened kept its full copy for as long as the tab was open.
The store now holds bodies for the 40 messages most recently wanted; older
ones go back to the list properties and are fetched in full again if
opened. The open conversation is never released.

Contact, settings and calendar exports go through downloadFile, which
releases the object URL once the download has started; three of them
never released it.

web/src/views/calendar/CalendarSidebar.tsx

@@ -16,6 +16,7 @@ import type { Calendar, Id } from "@/jmap/types";
 import { CalendarDialog } from "./CalendarDialog";
 import { ShareDialog } from "../settings/ShareDialog";
 import { plural, t } from "@/lib/i18n";
+import { downloadFile } from "@/lib/download";
 
 export function CalendarSidebar() {
   const [location, navigate] = useLocation();
@@ -42,19 +43,14 @@ export function CalendarSidebar() {
   const importInto = useRef<Id | null>(null);
 
   /*
-   * Handing the file over, which the browser only does from a click. The
-   * revoke below is what keeps a calendar's worth of text from sitting in
-   * memory after the download has started.
+   * Handing the file over, which the browser only does from a click.
+   * `downloadFile` releases it once started, so a calendar's worth of text
+   * does not sit in memory afterwards.
    */
   const exportFile = async (c: Calendar) => {
     try {
       const { text, count } = await cal.exportIcs(c.id);
-      const url = URL.createObjectURL(new Blob([text], { type: "text/calendar" }));
-      const a = document.createElement("a");
-      a.href = url;
-      a.download = `${c.name.replace(/[^\w.-]+/g, "_") || "calendar"}.ics`;
-      a.click();
-      URL.revokeObjectURL(url);
+      downloadFile(text, "text/calendar", `${c.name.replace(/[^\w.-]+/g, "_") || "calendar"}.ics`);
       toast.success(plural(count, { one: "Exported {n} event", other: "Exported {n} events" }));
     } catch (err) {
       toast.error(t("Could not export this calendar: {error}", { error: (err as Error).message }));