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/features/toast/LotusToastContainer.tsx

@@ -171,7 +171,11 @@ function ToastCard({ toast }: ToastCardProps) {
       onKeyDown={(e) => {
         if (e.key === 'Enter' || e.key === ' ') handleCardClick();
       }}
-      aria-label={`Notification from ${toast.displayName} in ${toast.roomName}`}
+      aria-label={
+        toast.roomName
+          ? `Notification from ${toast.displayName} in ${toast.roomName}`
+          : `${toast.displayName}: ${toast.body}`
+      }
     >
       <span style={{ position: 'absolute', top: config.space.S100, right: config.space.S100 }}>
         <IconButton
@@ -187,7 +191,11 @@ function ToastCard({ toast }: ToastCardProps) {
         </IconButton>
       </span>
       <div style={rowStyle}>
-        {toast.avatarUrl ? (
+        {toast.iconSrc ? (
+          <div style={initialsStyle} aria-hidden="true">
+            <Icon size="100" src={toast.iconSrc} />
+          </div>
+        ) : toast.avatarUrl ? (
           <img src={toast.avatarUrl} alt="" style={avatarStyle} aria-hidden="true" />
         ) : (
           <div style={initialsStyle} aria-hidden="true">
@@ -197,7 +205,7 @@ function ToastCard({ toast }: ToastCardProps) {
         <span style={nameStyle}>{toast.displayName}</span>
       </div>
       <div style={bodyStyle}>{toast.body}</div>
-      <div style={roomNameStyle}>{toast.roomName}</div>
+      {toast.roomName && <div style={roomNameStyle}>{toast.roomName}</div>}
     </div>
   );
 }