feat: show reading progress in Explore and Bookmarks sidebar

- Add reading position loading to Explore component
- Add reading position loading to useBookmarksData hook
- Display progress bars in Explore tab blog posts
- Display progress bars in Bookmarks large preview view
- Progress shown as colored bar (green for completed, orange for in-progress)
- Only shown for kind:30023 articles with saved reading positions
- Requires syncReadingPosition setting to be enabled

src/components/ThreePaneLayout.tsx

@@ -47,6 +47,7 @@ interface ThreePaneLayoutProps {
   onRefresh: () => void
   relayPool: RelayPool | null
   eventStore: IEventStore | null
+  readingPositions?: Map<string, number>
   
   // Content pane
   readerLoading: boolean
@@ -324,6 +325,7 @@ const ThreePaneLayout: React.FC<ThreePaneLayoutProps> = (props) => {
             loading={props.bookmarksLoading}
             relayPool={props.relayPool}
             isMobile={isMobile}
+            readingPositions={props.readingPositions}
             settings={props.settings}
           />
         </div>