feat: route custom token metadata imports through proxy

[0xApotheosis]

Summary

• route custom token metadata import lookups through api.proxy.shapeshift.com instead of direct browser Alchemy/Metaplex calls
• remove dead client-side Alchemy SDK usage and centralize supported custom-token chain IDs
• remove committed VITE_ALCHEMY_POLYGON_URL and related config/type references, and update CSP for proxy-based metadata lookups

Testing

• yarn eslint "src/components/TradeAssetSearch/hooks/useGetCustomTokensQuery.tsx" "src/components/TradeAssetSearch/components/SearchTermAssetList.tsx" "src/components/Layout/Header/GlobalSearch/GlobalSearchModal.tsx" "src/lib/customTokenImportSupportedChainIds.ts" "src/config.ts" "src/vite-env.d.ts" "headers/csps/customTokenImport.ts" "headers/csps/chains/ethereum.ts"
• manual local metadata query against proxy endpoint (ETH USDC)

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/custom-token-metadata-proxy-v2

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