feat: add inline keyboard, /start command, and callback handlers

- Show main menu with Clock In/Out, Report, Export, Remote, Day Off buttons
- Each action result includes a 'Back to Menu' button
- Handle callback queries alongside text commands
- Delete stale webhook before switching to polling
- Log i18n initialization error instead of silently ignoring it

cmd/bot/webhook.go

@@ -57,5 +57,8 @@ func startWebhook(botAPI *tgbotapi.BotAPI, handler *bot.Handler, webhookURL stri
 		if update.Message != nil {
 			handler.HandleMessage(update)
 		}
+		if update.CallbackQuery != nil {
+			handler.HandleCallback(update)
+		}
 	}
 }