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/lib/journal.ts

@@ -60,3 +60,23 @@ export function monthGrid(year: number, month: number): (string | null)[][] {
   for (let i = 0; i < cells.length; i += 7) weeks.push(cells.slice(i, i + 7));
   return weeks;
 }
+
+// Workspace state for this browser tab: is the journal open, which regular note was open last
+const WORKSPACE_SESSION_KEY = 'mindsift_workspace_view';
+
+export function readWorkspaceSession(): { journal: boolean; docId: string | null } {
+  try {
+    const raw = JSON.parse(sessionStorage.getItem(WORKSPACE_SESSION_KEY) || '{}');
+    return { journal: raw.journal === true, docId: typeof raw.docId === 'string' ? raw.docId : null };
+  } catch {
+    return { journal: false, docId: null };
+  }
+}
+
+export function writeWorkspaceSession(state: { journal: boolean; docId: string | null }) {
+  try {
+    sessionStorage.setItem(WORKSPACE_SESSION_KEY, JSON.stringify(state));
+  } catch {
+    // storage blocked: the view simply starts fresh
+  }
+}