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.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