WalletConnect buy: lower maxFeePerGas ceiling to 50 gwei

250 gwei ceiling tripped MetaMask's "network fee higher than necessary" alert on
Amoy (base fee ~0). 50 gwei keeps priority above Polygon's ~25 gwei floor while
reading as a normal fee, so buyers get a clean Confirm.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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