feat(workspace): merge bullet on Backspace, render hashtags inline

- Backspace at the start of a non-empty bullet appends its text to the
  previous visible bullet (skipped for blocks with children and for
  code/divider/table/progress blocks).
- Hashtags are rendered inline as clickable chips instead of a badge row
  below the text, so lines no longer grow taller.
- Targeted/highlighted bullets use a neutral background instead of accent.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

frontend/src/features/workspace/OutlinerTree.tsx

@@ -50,27 +50,34 @@ export const OutlinerTree = forwardRef<OutlinerTreeHandle, OutlinerTreeProps>(({
   const [dragOverIndex, setDragOverIndex] = useState<number | null>(null);
 
   const textareaMapRef = useRef<Map<string, HTMLTextAreaElement>>(new Map());
-  const pendingFocusRef = useRef<{ blockId: string; cursor: 'start' | 'end' | 'select' } | null>(null);
+  const pendingFocusRef = useRef<{ blockId: string; cursor: 'start' | 'end' | 'select' | number } | null>(null);
 
   const focusBlockInput = useCallback(
-    (blockId: string, cursor: 'start' | 'end' | 'select' = 'end') => {
+    (blockId: string, cursor: 'start' | 'end' | 'select' | number = 'end') => {
       setFocusedBlockId(blockId);
       const el = textareaMapRef.current.get(blockId);
       if (el) {
-        requestAnimationFrame(() => {
+        const apply = () => {
           el.focus();
           try {
             if (cursor === 'select') {
               el.select();
             } else {
-              const pos = cursor === 'end' ? el.value.length : 0;
+              const pos =
+                typeof cursor === 'number'
+                  ? Math.min(cursor, el.value.length)
+                  : cursor === 'end'
+                  ? el.value.length
+                  : 0;
               el.setSelectionRange(pos, pos);
             }
           } catch {
             // ignore
           }
           el.scrollIntoView({ behavior: 'smooth', block: 'center' });
-        });
+        };
+        // A numeric position refers to content that was just changed, so wait one extra frame for it to render
+        requestAnimationFrame(() => (typeof cursor === 'number' ? requestAnimationFrame(apply) : apply()));
       } else {
         pendingFocusRef.current = { blockId, cursor };
       }
@@ -514,6 +521,30 @@ export const OutlinerTree = forwardRef<OutlinerTreeHandle, OutlinerTreeProps>(({
     [blocks, canEdit, focusBlockInput, levelOffset, onDeleteBlock, onUpdateBlock]
   );
 
+  // Backspace at the start of a non-empty block: append its text to the previous visible block
+  const handleMergeWithPrevious = useCallback(
+    (blockId: string, content: string) => {
+      if (!canEdit) return false;
+      const mergeable = ['text', 'paragraph', 'bullet', 'todo', 'h1', 'h2', 'h3', 'callout'];
+      const visIdx = visibleEntries.findIndex((e) => e.block.id === blockId);
+      if (visIdx <= 0) return false;
+      const prev = visibleEntries[visIdx - 1].block;
+      const idx = blocks.findIndex((b) => b.id === blockId);
+      const cur = blocks[idx];
+      if (!cur || !mergeable.includes(prev.type) || !mergeable.includes(cur.type)) return false;
+      // Blocks with children would leave them orphaned at a wrong indent level
+      const next = blocks[idx + 1];
+      if (next && (next.level || 0) > (cur.level || 0)) return false;
+
+      const joinPos = (prev.content || '').length;
+      onUpdateBlock({ ...prev, content: (prev.content || '') + content }, true);
+      onDeleteBlock(cur.id);
+      focusBlockInput(prev.id, joinPos);
+      return true;
+    },
+    [blocks, canEdit, focusBlockInput, onDeleteBlock, onUpdateBlock, visibleEntries]
+  );
+
   const handleIndentBlock = useCallback(
     (blockId: string, delta: 1 | -1) => {
       if (!canEdit) return;
@@ -672,6 +703,7 @@ export const OutlinerTree = forwardRef<OutlinerTreeHandle, OutlinerTreeProps>(({
             onCreateBlockBelow={handleCreateBlockBelow}
             onPasteMultiLines={handlePasteMultiLines}
             onDeleteBlock={handleDeleteBlock}
+            onMergeWithPrevious={handleMergeWithPrevious}
             onIndentBlock={handleIndentBlock}
             onNavigateFocus={handleNavigateFocus}
             onFocusBlock={setFocusedBlockId}