feat(workspace): journal with one note per day and a calendar

- journal_days (migration 8) maps a person's day to an ordinary note;
  GET /journal lists the days, POST /journal/{date} opens or creates one
- the journal sits on top of the notes sidebar: days below each other
  (today first, older ones load while scrolling), a calendar with month
  and year picker follows the day in view and jumps to any day
- the header shares the day in view; journal notes stay out of the folder
  tree; can be switched off per device under Darstellung

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

frontend/src/lib/__tests__/journal.test.ts

@@ -0,0 +1,16 @@
+import { describe, it, expect } from 'vitest';
+import { monthGrid, toISODate, parseISODate } from '../journal';
+
+describe('journal dates', () => {
+  it('builds a Monday-first month grid', () => {
+    // October 2026 starts on a Thursday
+    const weeks = monthGrid(2026, 9);
+    expect(weeks[0]).toEqual([null, null, null, '2026-10-01', '2026-10-02', '2026-10-03', '2026-10-04']);
+    expect(weeks.flat().filter(Boolean)).toHaveLength(31);
+    expect(weeks.every((w) => w.length === 7)).toBe(true);
+  });
+
+  it('round-trips local dates', () => {
+    expect(toISODate(parseISODate('2026-03-01'))).toBe('2026-03-01');
+  });
+});