feat: add manual interval creation and inline entry editing

- Service: CreateInterval() validates same-day, end>start, day not closed
- Service: Update() now rejects edits on closed-day entries (ErrDayAlreadyClosed)
- Handler: POST /api/entries creates a completed interval with explicit times
- API client: entries.createInterval(startMs, endMs, note)
- Utils: parseTimeInput / toTimeInput helpers for HH:MM <-> unix ms
- Today page: '+ Add interval' form (time pickers + optional note)
- Today page: pencil button on each entry opens inline edit row (start/end/note)
- Tests: TestCreateInterval, TestCreateIntervalEndBeforeStart,
         TestCreateIntervalCrossesMidnight, TestUpdateRejectsClosedDay

internal/service/entry_service.go

@@ -176,6 +176,15 @@ func (s *EntryService) Update(ctx context.Context, id string, input UpdateEntryI
 		return nil, err
 	}
 
+	// Disallow editing entries that belong to a closed day.
+	closed, err := s.closedDays.GetByDayKey(ctx, e.DayKey)
+	if err != nil && !errors.Is(err, sql.ErrNoRows) {
+		return nil, err
+	}
+	if closed != nil {
+		return nil, ErrDayAlreadyClosed
+	}
+
 	if input.StartTime != nil {
 		e.StartTime = *input.StartTime
 		e.DayKey = s.dayKeyForMs(e.StartTime)
@@ -203,6 +212,50 @@ func (s *EntryService) Update(ctx context.Context, id string, input UpdateEntryI
 	return e, nil
 }
 
+// CreateIntervalInput holds fields for a manually created completed interval.
+type CreateIntervalInput struct {
+	StartTime int64
+	EndTime   int64
+	Note      string
+}
+
+// CreateInterval adds a completed interval with explicit start and end times.
+// Rules: same-day, end > start, day not closed, no currently running entry on that day (check running globally).
+func (s *EntryService) CreateInterval(ctx context.Context, input CreateIntervalInput) (*domain.Entry, error) {
+	if input.EndTime <= input.StartTime {
+		return nil, fmt.Errorf("end_time must be after start_time")
+	}
+
+	startDayKey := s.dayKeyForMs(input.StartTime)
+	endDayKey := s.dayKeyForMs(input.EndTime)
+	if startDayKey != endDayKey {
+		return nil, ErrCrossesMidnight
+	}
+
+	// Check day is not closed.
+	closed, err := s.closedDays.GetByDayKey(ctx, startDayKey)
+	if err != nil && !errors.Is(err, sql.ErrNoRows) {
+		return nil, err
+	}
+	if closed != nil {
+		return nil, ErrDayAlreadyClosed
+	}
+
+	nowMs := s.nowMs()
+	e := &domain.Entry{
+		ID:        uuid.New().String(),
+		StartTime: input.StartTime,
+		EndTime:   &input.EndTime,
+		Note:      input.Note,
+		DayKey:    startDayKey,
+		UpdatedAt: nowMs,
+	}
+	if err := s.entries.Create(ctx, e); err != nil {
+		return nil, err
+	}
+	return e, nil
+}
+
 // Delete soft-deletes an entry.
 func (s *EntryService) Delete(ctx context.Context, id string) error {
 	nowMs := s.nowMs()