Can anyone advise?

A couple of months ago I bought a gorgeous full flowering french
lavender (papillon?) from my local garden centre but all the flowers
have now died. I thought it was meant to last the summer?

To show my ignorance here, was I meant to cut out the dead flowers to
promote new growth?

If anyone can give me some tips for caring for this plant so I don't
make the same mistake next year (assuming it lasts that long!), I would
be most grateful.

Many thanks
Rebecca