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/api/server.go

@@ -217,6 +217,8 @@ func (s *Server) Router() http.Handler {
 		r.Get("/api/v1/workspace/tags/{tag}", s.handleGetTagDetails)
 		r.Put("/api/v1/workspace/tags/{tag}/note", s.handleUpdateTagPageNote)
 
+		r.Get("/api/v1/journal", s.handleGetJournal)
+		r.Post("/api/v1/journal/{date}", s.handleOpenJournalDay)
 		r.Get("/api/v1/documents", s.handleGetDocuments)
 		r.Post("/api/v1/documents", s.handleCreateDocument)
 		r.Get("/api/v1/documents/{id}", s.handleGetDocument)