fix: sweep remaining hardcoded Sepolia labels in the privacy lab

Confirmed on mainnet today: registration succeeded through the browser
(0x21b12f4d..., SUCCEEDED, 4.42 STRK, get_public_key now non-zero for
0x06c90d9b...887a9) - the pool address and 6 STRK fee shown in the UI were
already correctly mainnet throughout. Only the button read "Sign and register
on Sepolia" regardless of network.

The earlier network-aware pass verified itself with a case-sensitive grep for
"sepolia", which missed every capitalized mid-sentence "Sepolia" - five in
strk20-registration-lab.tsx (error text, two status messages, two button
labels), two in strk20-shield-lab.tsx, two in strk20-usdc-lab.tsx. All now
read from the same networkLabel each component already computes. Also dropped
the `constants` import all three left unused after chainId moved to
sdk.starknetChainId.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>