fix: address audit findings — nil pointers, timezone bug, dead code, tests, README

- Fix nil pointer dereference in clock.go (guard GetOrCreateSettings and
  GetOrCreateRPGStats errors before accessing fields)
- Fix timezone bug in handleSetBreakMsg (was using system time.Now()
  instead of user's configured timezone)
- Remove dead code: sendExportMonthPicker (unused wrapper)
- Simplify xpForWorkSeconds (xpPerSecond=1, inline to just return sec)
- Extract computeStreakFromDates as pure testable function from
  recalcAggregates; add 6 tests covering empty, current, gaps,
  no-today, long streak, longest-not-current edge cases
- Update README with full feature documentation: RPG, League, Salary,
  Burnout, Work Types, Inline History Editing, all missing commands

internal/bot/export.go

@@ -388,11 +388,6 @@ func (h *Handler) exportCallback(ctx context.Context, chatID int64, msgID int, c
 	h.editExportMonthPicker(ctx, chatID, msgID, cy, cm, user)
 }
 
-// sendExportMonthPicker sends the export month picker as a new message.
-func (h *Handler) sendExportMonthPicker(ctx context.Context, chatID int64, msgID int, year, month int, user *db.User) {
-	h.editExportMonthPicker(ctx, chatID, msgID, year, month, user)
-}
-
 // editExportMonthPicker renders a year-based month picker with 12 month buttons.
 func (h *Handler) editExportMonthPicker(ctx context.Context, chatID int64, msgID int, year, _ int, user *db.User) {
 	months := gregMonthNames