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/main.go

@@ -87,6 +87,7 @@ func main() {
 	dbStore.Close()
 }
 
+// startPolling runs the long-polling update loop, dispatching messages and callbacks to the handler.
 func startPolling(botAPI *tgbotapi.BotAPI, handler *bot.Handler, stop chan struct{}) {
 	u := tgbotapi.NewUpdate(0)
 	u.Timeout = 30
@@ -109,6 +110,7 @@ func startPolling(botAPI *tgbotapi.BotAPI, handler *bot.Handler, stop chan struc
 	}
 }
 
+// parseAllowedUsers parses a comma-separated list of Telegram user IDs into a set.
 func parseAllowedUsers(raw string) map[int64]bool {
 	m := make(map[int64]bool)
 	for _, s := range strings.Split(raw, ",") {
@@ -126,6 +128,7 @@ func parseAllowedUsers(raw string) map[int64]bool {
 	return m
 }
 
+// dailyReportScheduler runs a periodic ticker that triggers daily report checks every 30 seconds.
 func dailyReportScheduler(h *bot.Handler, store *db.Store, stop chan struct{}) {
 	ticker := time.NewTicker(30 * time.Second)
 	defer ticker.Stop()
@@ -140,6 +143,7 @@ func dailyReportScheduler(h *bot.Handler, store *db.Store, stop chan struct{}) {
 	}
 }
 
+// checkDailyReports iterates all users and sends a daily report to those whose report time has arrived.
 func checkDailyReports(h *bot.Handler, store *db.Store) {
 	users, err := store.GetAllUsers()
 	if err != nil {
@@ -177,6 +181,7 @@ func checkDailyReports(h *bot.Handler, store *db.Store) {
 	}
 }
 
+// getEnvDefault returns the environment variable value or a default if unset or empty.
 func getEnvDefault(key, defaultValue string) string {
 	if val := os.Getenv(key); val != "" {
 		return val