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>

internal/storage/migrations.go

@@ -28,6 +28,7 @@ var migrations = []migration{
 	{5, "Medien", (*Store).migrateMedia},
 	{6, "Link-Vorschauen", (*Store).migrateLinkPreviews},
 	{7, "Benachrichtigungen", (*Store).migrateNotifications},
+	{8, "Journal", (*Store).migrateJournal},
 }
 
 func (s *Store) migrate() error {