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/settings/GeneralSettings.tsx

@@ -25,6 +25,7 @@ import {
   type DateFormat,
 } from "@/lib/datetime";
 import { isEnforced } from "@/lib/settingsPolicy";
+import { downloadFile } from "@/lib/download";
 
 /** Illustrative instant used for the format previews: 22 Nov 2025, 18:23. */
 const SAMPLE = new Date(2025, 10, 22, 18, 23);
@@ -236,7 +237,7 @@ export function GeneralSettings() {
 
       <h2>{t("Backup")}</h2>
       <div className="row wrap">
-        <button className="btn" onClick={() => { const blob = new Blob([exportJson()], { type: "application/json" }); const a = document.createElement("a"); a.href = URL.createObjectURL(blob); a.download = "ihasmail-settings.json"; a.click(); }}>{t("Export settings")}</button>
+        <button className="btn" onClick={() => downloadFile(exportJson(), "application/json", "ihasmail-settings.json")}>{t("Export settings")}</button>
         <label className="btn">
           {t("Import settings")}
           <input type="file" accept="application/json" hidden onChange={async (e) => { const f = e.target.files?.[0]; if (!f) return; const ok = importJson(await f.text()); toast[ok ? "success" : "error"](ok ? t("Settings imported") : t("Invalid settings file")); e.target.value = ""; }} />