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)

internal/bot/totals.go

@@ -1,3 +1,5 @@
+// Package bot implements the Telegram bot logic: message routing, time tracking,
+// calendar rendering, Excel export, and inline editing of events.
 package bot
 
 import (
@@ -6,19 +8,27 @@ import (
 	"worktimeBot/internal/db"
 )
 
+// State represents the current tracking state during daily totals computation.
 type State int
 
 const (
+	// StateIdle indicates no active work session.
 	StateIdle State = iota
+	// StateWorking indicates an active work session.
 	StateWorking
+	// StateOnBreak indicates an active break session.
 	StateOnBreak
 )
 
+// DailyTotals holds the computed total work and break seconds for a single day.
 type DailyTotals struct {
 	TotalSeconds int64
 	BreakSeconds int64
 }
 
+// ComputeDailyTotals calculates the total work and break time from a list of events.
+// It sorts events chronologically, inserts synthetic start/end events when needed,
+// and tracks state transitions between idle, working, and on-break.
 func ComputeDailyTotals(events []db.Event, minBreakThreshold int64, dayStart, dayEnd int64) DailyTotals {
 	if len(events) == 0 {
 		return DailyTotals{}