Do I lose voting power when I stake my $PACT?

No. Let's say you have 10K $PACT in your wallet. You decide to stake it.

When doing so, you are sending the 10k $PACT from your wallet to the staking contract, and in return, the staking contract gives you 10k SPACT. This new token, SPACT can be used for voting. This is mainly to prevent you from losing voting power.

At any moment, you can unstake. When unstaking, the staking contract burns your SPACT and gives you your 10K PACT again.