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/journal.go

@@ -0,0 +1,70 @@
+package api
+
+import (
+	"fmt"
+	"net/http"
+	"sync"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/sandrodach/mindsift/internal/auth"
+)
+
+// Journal: one ordinary note per day, opened (and created on first use) by date. The web app shows the
+// days below each other with a calendar; the notes can be shared and searched like any other.
+
+var journalMu sync.Mutex // one creation at a time, so a day never gets two notes
+
+var (
+	journalWeekdays = []string{"Sonntag", "Montag", "Dienstag", "Mittwoch", "Donnerstag", "Freitag", "Samstag"}
+	journalMonths   = []string{"Januar", "Februar", "März", "April", "Mai", "Juni", "Juli", "August", "September", "Oktober", "November", "Dezember"}
+)
+
+// journalTitle: "Freitag, 30. Oktober 2026"
+func journalTitle(day time.Time) string {
+	return fmt.Sprintf("%s, %d. %s %d", journalWeekdays[day.Weekday()], day.Day(), journalMonths[day.Month()-1], day.Year())
+}
+
+func (s *Server) handleGetJournal(w http.ResponseWriter, r *http.Request) {
+	days, err := s.store.JournalDays(auth.GetUserID(r.Context()))
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "Journal konnte nicht geladen werden")
+		return
+	}
+	writeJSON(w, http.StatusOK, map[string]interface{}{"days": days})
+}
+
+// handleOpenJournalDay returns the note of a day and creates it when there is none yet.
+func (s *Server) handleOpenJournalDay(w http.ResponseWriter, r *http.Request) {
+	userID := auth.GetUserID(r.Context())
+	date := chi.URLParam(r, "date")
+	day, err := time.Parse("2006-01-02", date)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "Ungültiges Datum (erwartet JJJJ-MM-TT)")
+		return
+	}
+	journalMu.Lock()
+	defer journalMu.Unlock()
+	docID, err := s.store.JournalDocID(userID, date)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "Journal konnte nicht geladen werden")
+		return
+	}
+	if docID != "" {
+		if doc, err := s.store.GetDocumentByID(userID, docID); err == nil {
+			writeJSON(w, http.StatusOK, doc)
+			return
+		}
+	}
+	doc, err := s.store.CreateDocument(userID, "", journalTitle(day), "📅", "", nil, nil, false)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "Journal-Tag konnte nicht angelegt werden")
+		return
+	}
+	if err := s.store.SetJournalDay(userID, date, doc.ID); err != nil {
+		writeError(w, http.StatusInternalServerError, "Journal-Tag konnte nicht angelegt werden")
+		return
+	}
+	s.broadcastDocToAuthorizedUsers(doc.ID, "DOC_CREATED")
+	writeJSON(w, http.StatusCreated, doc)
+}