feat(download): show a toast + button check when a file is saved

The desktop (Tauri) app has no native download UI, so FileSaver.saveAs saved
files silently — no visual or audio confirmation. Users re-clicked because
nothing said it worked (one report: 5 copies of the same file). Add a small
useSaveFile() hook that saves AND raises a 'Downloaded <filename>' toast, and
route every download call site through it (file attachments, image viewer, PDF
viewer, plus the recovery-key / key-backup exports). The file-message download
button also shows a green check on success.

Toast system extended with an optional iconSrc so system toasts render an icon
instead of an avatar/initials, and an empty roomName is no longer rendered.

Tests: createDownloadToast covered; 701/701 pass; typecheck + build clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/state/toast.test.ts

@@ -1,7 +1,7 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
 import { createStore } from 'jotai';
-import { toastQueueAtom, dismissToastAtom, ToastNotif } from './toast';
+import { toastQueueAtom, dismissToastAtom, ToastNotif, createDownloadToast } from './toast';
 
 // The queue lives in an unexported baseAtom; we drive the two write-only setters
 // (toastQueueAtom append + null no-op guard, dismissToastAtom remove-by-id)
@@ -85,3 +85,15 @@ test('dismissToastAtom for an unknown id is a no-op', () => {
     ['a'],
   );
 });
+
+test('createDownloadToast: filename in body, no room navigation, unique ids', () => {
+  const a = createDownloadToast('photo.jpg');
+  assert.equal(a.displayName, 'Downloaded');
+  assert.equal(a.body, 'photo.jpg');
+  // roomId empty + an onClick present → clicking dismisses without navigating to a room.
+  assert.equal(a.roomId, '');
+  assert.equal(a.roomName, '');
+  assert.equal(typeof a.onClick, 'function');
+  const b = createDownloadToast('photo.jpg');
+  assert.notEqual(a.id, b.id);
+});

src/app/state/toast.ts

@@ -1,8 +1,10 @@
 import { atom } from 'jotai';
+import type { IconSrc } from 'folds';
 
 export type ToastNotif = {
   id: string;
   avatarUrl?: string;
+  iconSrc?: IconSrc; // folds Icon src for a "system" toast (shown instead of an avatar/initials)
   displayName: string;
   body: string;
   roomName: string;
@@ -12,6 +14,19 @@ export type ToastNotif = {
   sticky?: boolean; // when true, does not auto-dismiss — use for action toasts that require a click
 };
 
+// Build a "download complete" system toast. Kept folds-free here (the icon src is
+// passed in) so this stays a pure, testable builder. roomId is empty + onClick is
+// set so a click only dismisses (never navigates to a room).
+export const createDownloadToast = (filename: string, iconSrc?: IconSrc): ToastNotif => ({
+  id: `download-${Date.now()}-${Math.random().toString(36).slice(2, 8)}`,
+  displayName: 'Downloaded',
+  body: filename,
+  roomName: '',
+  roomId: '',
+  iconSrc,
+  onClick: () => undefined,
+});
+
 const baseAtom = atom<ToastNotif[]>([]);
 
 // Write-only setter used in ClientNonUIFeatures