Add NoOp email service, /docs shortcut and fix streak timezone bug

Replace nil email service pattern with NoOpEmailService (Null Object)
to eliminate nil pointer panics across all handlers.

Add /docs route as a shortcut to Swagger UI.

Fix streak calculation returning 0 when server timezone differs from
UTC — CreatedAt was not converted to UTC before date extraction.

internal/domain/services/streak.go

@@ -49,7 +49,8 @@ func buildEntryMap(entries []*entities.HabitEntry) map[string]*entities.HabitEnt
 }
 
 func allScheduledDates(habit *entities.Habit, now time.Time) []time.Time {
-	start := time.Date(habit.CreatedAt.Year(), habit.CreatedAt.Month(), habit.CreatedAt.Day(), 0, 0, 0, 0, time.UTC)
+	createdUTC := habit.CreatedAt.UTC()
+	start := time.Date(createdUTC.Year(), createdUTC.Month(), createdUTC.Day(), 0, 0, 0, 0, time.UTC)
 	today := time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, time.UTC)
 	freq := string(habit.Frequency)