feat(content): add Images content type with masonry grid layout

Extract markdown images and raw image URLs from AI responses into a
browsable image gallery. Masonry (columns) layout in the grid,
full-size detail view with source links. Also fix ContextLoader to
accept all content types for loading state display.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/core/src/types/content.ts

@@ -125,3 +125,15 @@ export interface BookSource {
   url: string
   icon?: string
 }
+
+export interface ImageItem {
+  id: string
+  url: string
+  title?: string
+  description?: string
+  alt?: string
+  width?: number
+  height?: number
+  source?: string
+  attribution?: string
+}