feat: added demo timer_constructor; docs: added db in the .gitignore

kv/timer_screen.kv

@@ -32,6 +32,6 @@
         # Label для отображения времени
         Label:
             id: time_label
-            text: '00:00:12'
+            # text: root.get_init_time(self)
             size_hint: None, None
             pos_hint: {'center_x': 0.5, 'center_y': 0.6}