* FTSEurofirst 300 up 1.4 percent * Euro STOXX 50 volatility index falls 5 pct * Euro banks advance, UK banks underperform By Tricia Wright LONDON, June 29 (Reuters) - European shares jumped to one-week highs on Friday after euro zone leaders agreed to take action to bring down Italy's and Spain's borrowing costs, surprising a market that had been primed for disappointment from their two-day summit. The FTSEurofirst 300 rose 1.4 percent to 1,009.20, after falling 0.5 percent