Fix proxy miner diff calculation and unify proxy target handling in miner diff logic

[MoneroOcean]

Motivation

• Correct proxy miner hash-rate detection which previously mixed milliseconds and seconds and used a raw hashes/time comparison that could be incorrect.
• Ensure proxy detection compares a properly computed proxy hash-rate against miner.difficulty and avoids division-by-zero.
• Consolidate the hardcoded proxy target value into a single proxyTarget for consistent retarget behavior.

Description

• Introduced a proxyTarget constant and compute proxyDiff as proxyMiner.hashes * proxyTarget / proxyPeriod with proxyPeriod in seconds and guarded by proxyPeriod > 0.
• Switched proxy detection to if (proxyMiner && proxyDiff > miner.difficulty) and use proxyTarget when assigning target for proxy-related branches and throttled wallets.
• Updated the miner.proxy branch to use proxyTarget as the target and left existing minDiff and historyTime logic intact while initializing proxyDiff to zero.

Testing

• Ran unit tests with npm test and they passed.
• Ran linter with npm run lint and it passed.
• No automated test failures were observed.

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Codex Task