refactor: split handlers.go into 5 files, redesign export month picker, pad calendar rows, improve comments

- Split 1571-line handlers.go into: handlers.go (core), clock.go, settings.go, calendar.go, report.go
- Redesigned export month picker: year navigation + 12-month grid instead of prev/next month
- Fixed calendar last row: pad remaining cells with empty buttons to ensure 7 columns
- Added Go doc comments across all files (dateutil.go, totals.go, store.go, main.go, webhook.go)

cmd/bot/webhook.go

@@ -10,6 +10,7 @@ import (
 	"worktimeBot/internal/bot"
 )
 
+// startWebhook registers the webhook with Telegram and starts the HTTPS/HTTP listener loop.
 func startWebhook(botAPI *tgbotapi.BotAPI, handler *bot.Handler, webhookURL string) {
 	if _, err := botAPI.Request(tgbotapi.DeleteWebhookConfig{}); err != nil {
 		slog.Error("delete webhook", "error", err)