feat(workspace): journal panel toggle, view kept across tabs, sections nest under headings

- the journal opens as a framed panel and closes on a second click,
  returning to the previous note; journal state and the last regular note
  survive tab switches, the fallback note is never a journal day
- pasted and imported markdown nests content under its heading (H2 below
  H1), Enter on a heading creates a child, '- ' / '* ' / '+ ' start a bullet

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

frontend/src/features/workspace/OutlinerTree.tsx

@@ -247,7 +247,11 @@ export const OutlinerTree = forwardRef<OutlinerTreeHandle, OutlinerTreeProps>(({
       if (!canEdit) return;
       const idx = blocks.findIndex((b) => b.id === afterBlockId);
       const currentBlock = idx >= 0 ? blocks[idx] : null;
-      const level = currentBlock ? Math.max(0, currentBlock.level || 0) : 0;
+      const isHeading =
+        (currentBlock?.type === 'h1' || currentBlock?.type === 'h2' || currentBlock?.type === 'h3') &&
+        !currentBlock.collapsed;
+      // Enter on an open heading starts its section: the new bullet goes below it as a child
+      const level = currentBlock ? Math.max(0, currentBlock.level || 0) + (isHeading ? 1 : 0) : 0;
       // Numbered lists continue on Enter (an empty item ends the list, see OutlinerBlock)
       const inheritedType: BlockType = currentBlock?.type === 'numbered' ? 'numbered' : 'text';