fix: calendar day-of-week offset off by one, nav double-jump, race conditions

- Fix Jalali/Gregorian/Hijri day-of-week startOffset formulas
  (dow returns 0=Sat, not 0=Sun as the old comment claimed)
- Fix calendar and export month picker navigation jumping 2 months
  (navMonth buttons already have pre-computed values; handlers
  were decrementing/incrementing them again)
- Fix GetOrCreateUser/GetOrCreateDay race condition with INSERT OR IGNORE
- Add month/day bounds validation in handleEditMsg to prevent panic

internal/db/store.go

@@ -289,15 +289,8 @@ func migrateSettings(db *sql.DB) {
 }
 
 func (s *Store) GetOrCreateUser(chatID int64) (*User, error) {
-	user, err := s.GetUserByChatID(chatID)
-	if err == nil {
-		return user, nil
-	}
-	if err != sql.ErrNoRows {
-		return nil, err
-	}
-	_, err = s.db.Exec(
-		"INSERT INTO users (chat_id) VALUES (?)",
+	_, err := s.db.Exec(
+		"INSERT OR IGNORE INTO users (chat_id) VALUES (?)",
 		chatID,
 	)
 	if err != nil {
@@ -399,15 +392,8 @@ func (s *Store) GetWorkType(id int64) (*WorkType, error) {
 }
 
 func (s *Store) GetOrCreateDay(userID int64, date string) (*Day, error) {
-	day, err := s.GetDay(userID, date)
-	if err == nil {
-		return day, nil
-	}
-	if err != sql.ErrNoRows {
-		return nil, err
-	}
-	_, err = s.db.Exec(
-		"INSERT INTO days (user_id, date) VALUES (?, ?)",
+	_, err := s.db.Exec(
+		"INSERT OR IGNORE INTO days (user_id, date) VALUES (?, ?)",
 		userID, date,
 	)
 	if err != nil {