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/journal.ts

@@ -0,0 +1,62 @@
+// Journal helpers: dates are local calendar days as YYYY-MM-DD (the server stores them as given).
+
+export const JOURNAL_ENABLED_KEY = 'mindsift_journal_enabled';
+
+export function isJournalEnabled(): boolean {
+  try {
+    return localStorage.getItem(JOURNAL_ENABLED_KEY) !== 'false';
+  } catch {
+    return true;
+  }
+}
+
+export function setJournalEnabled(on: boolean) {
+  try {
+    localStorage.setItem(JOURNAL_ENABLED_KEY, on ? 'true' : 'false');
+  } catch {
+    // storage blocked: the setting simply does not stick
+  }
+}
+
+export function toISODate(d: Date): string {
+  const m = String(d.getMonth() + 1).padStart(2, '0');
+  const day = String(d.getDate()).padStart(2, '0');
+  return `${d.getFullYear()}-${m}-${day}`;
+}
+
+export function todayISO(): string {
+  return toISODate(new Date());
+}
+
+export function parseISODate(date: string): Date {
+  const [y, m, d] = date.split('-').map(Number);
+  return new Date(y, (m || 1) - 1, d || 1);
+}
+
+export const MONTHS = [
+  'Januar', 'Februar', 'März', 'April', 'Mai', 'Juni',
+  'Juli', 'August', 'September', 'Oktober', 'November', 'Dezember',
+];
+
+/** "Fr, 30. Okt. 2026" for compact labels */
+export function shortDateLabel(date: string): string {
+  return parseISODate(date).toLocaleDateString('de-DE', {
+    weekday: 'short',
+    day: 'numeric',
+    month: 'short',
+    year: 'numeric',
+  });
+}
+
+/** Weeks of a month for a Monday-first grid; days outside the month are null. */
+export function monthGrid(year: number, month: number): (string | null)[][] {
+  const first = new Date(year, month, 1);
+  const offset = (first.getDay() + 6) % 7; // Monday = 0
+  const daysInMonth = new Date(year, month + 1, 0).getDate();
+  const cells: (string | null)[] = Array(offset).fill(null);
+  for (let d = 1; d <= daysInMonth; d++) cells.push(toISODate(new Date(year, month, d)));
+  while (cells.length % 7 !== 0) cells.push(null);
+  const weeks: (string | null)[][] = [];
+  for (let i = 0; i < cells.length; i += 7) weeks.push(cells.slice(i, i + 7));
+  return weeks;
+}