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/lib/download.ts

@@ -0,0 +1,16 @@
+/**
+ * Hand the browser a file the app made, to save.
+ *
+ * The object URL is released as soon as the download has been started: a
+ * click on the link starts it synchronously, and an unreleased URL keeps the
+ * whole file in memory for as long as the tab is open -- an address book's
+ * worth of vCards, per export.
+ */
+export function downloadFile(content: BlobPart, type: string, filename: string): void {
+  const url = URL.createObjectURL(new Blob([content], { type }));
+  const a = document.createElement("a");
+  a.href = url;
+  a.download = filename;
+  a.click();
+  URL.revokeObjectURL(url);
+}