InboxZone: a PixiJS Container positioned at (-500,-500) with a

dashed border and "Inbox" label. Auto-synced media slides in with
spring animations and random offset/rotation for a natural look.

MattermostImport: modal dialog for pulling images from a MM thread
URL and managing linked channels for auto-sync. Uses the existing
dark modal styling pattern from ShortcutsHelp.

Editor wiring: toolbar button for MM import, InboxZone added to
viewport on init, socket listener for board:media-arrived events
that routes assets into the InboxZone with a toast notification.

frontend/src/components/Toolbar.tsx

@@ -30,6 +30,7 @@ interface ToolbarProps {
   onToggleLayers?: () => void;
   showLayers?: boolean;
   onToggleHelp?: () => void;
+  onMmImport?: () => void;
   boardName?: string;
 }
 
@@ -142,6 +143,7 @@ export default function Toolbar({
   onToggleLayers,
   showLayers,
   onToggleHelp,
+  onMmImport,
 }: ToolbarProps) {
   const showStroke = activeTool === ToolType.PEN;
   const showFontSize = activeTool === ToolType.TEXT;
@@ -281,6 +283,16 @@ export default function Toolbar({
         </ActionBtn>
       )}
 
+      {/* Mattermost import */}
+      {onMmImport && (
+        <ActionBtn onClick={onMmImport} title="Import from Mattermost">
+          <svg width="14" height="14" viewBox="0 0 14 14" fill="none" stroke="currentColor" strokeWidth="1.3">
+            <path d="M2 10V4a2 2 0 012-2h6a2 2 0 012 2v6a2 2 0 01-2 2H4a2 2 0 01-2-2z" />
+            <path d="M5 7h4M7 5v4" strokeLinecap="round" />
+          </svg>
+        </ActionBtn>
+      )}
+
       {/* Spacer */}
       <div style={{ flex: 1 }} />