feat(workspace): split bulletpoint paragraph at cursor position on Enter

frontend/src/features/workspace/OutlinerBlock.tsx

@@ -55,7 +55,7 @@ export interface OutlinerBlockProps {
   levelOffset?: number;
   onZoomBlock?: (blockId: string) => void;
   onUpdateBlock: (updated: DocumentBlock, immediate?: boolean) => void;
-  onCreateBlockBelow: (afterBlockId: string, currentContent: string) => void;
+  onCreateBlockBelow: (afterBlockId: string, newContent?: string) => void;
   onDeleteBlock: (blockId: string, deleteSubtree?: boolean) => void;
   onIndentBlock: (blockId: string, delta: 1 | -1) => void;
   onNavigateFocus: (blockId: string, direction: 'up' | 'down') => void;
@@ -366,7 +366,7 @@ export const OutlinerBlock: React.FC<OutlinerBlockProps> = ({
     // 2. Enter (without Shift) -> If block is completely empty:
     //    - If level > minLevel, outdent (Shift+Tab behaviour)
     //    - If already at minLevel and is the last block, do NOT create another empty block below
-    //    - Otherwise create new block below
+    //    - Otherwise split paragraph at cursor and create new block below
     if (e.key === 'Enter' && !e.shiftKey && block.type !== 'code') {
       e.preventDefault();
       e.stopPropagation();
@@ -385,8 +385,27 @@ export const OutlinerBlock: React.FC<OutlinerBlockProps> = ({
         }
       }
 
-      flushDebounce(localContent);
-      onCreateBlockBelowRef.current(block.id, localContent);
+      const start = e.currentTarget.selectionStart ?? localContent.length;
+      const end = e.currentTarget.selectionEnd ?? start;
+      let beforeText = localContent.slice(0, start);
+      let afterText = localContent.slice(end);
+
+      // Clean linebreaks at the split boundary if present
+      if (beforeText.endsWith('\r\n')) {
+        beforeText = beforeText.slice(0, -2);
+      } else if (beforeText.endsWith('\n')) {
+        beforeText = beforeText.slice(0, -1);
+      }
+      if (afterText.startsWith('\r\n')) {
+        afterText = afterText.slice(2);
+      } else if (afterText.startsWith('\n')) {
+        afterText = afterText.slice(1);
+      }
+
+      setLocalContent(beforeText);
+      localContentRef.current = beforeText;
+      flushDebounce(beforeText);
+      onCreateBlockBelowRef.current(block.id, afterText);
       return;
     }