fix(tui): prevent End hint showing when already at bottom

The scroll_chat method was unconditionally setting chat_scroll_pinned_bottom
to false, which caused the '↓ End' hint to incorrectly appear when:
- Scrolling down while already at the bottom
- Pressing End key while at the bottom

Now we check if chat_scroll is 0 (at bottom) after the scroll operation
and keep pinned_bottom=true in that case.

Author: echobt

src/cortex-tui/src/app/state.rs

@@ -518,7 +518,9 @@ impl AppState {
         } else {
             self.chat_scroll = self.chat_scroll.saturating_add(delta as usize);
         }
-        self.chat_scroll_pinned_bottom = false;
+        // Only unpin from bottom if we actually scrolled away from it
+        // When chat_scroll is 0, we're at the bottom, so keep pinned
+        self.chat_scroll_pinned_bottom = self.chat_scroll == 0;
         self.show_scrollbar();
     }