ref: Set prefer-const eslint rule to error

[AbhiPrasad]

resolves #1409

AI Summary

js/eslint.config.ts: Changed "prefer-const": "warn" to "prefer-const": "error" (and removed the TODO comment).

js/src/logger.ts — 3 violations fixed:

1. Line 1773: Renamed spread variable to rawUpdateEvent and assigned filtered result to a new const updateEvent, since the original was being reassigned.
2. Line 5196: Changed for (let [name, score] to for (const [name, rawScore] with a let score = rawScore inside the loop body, since score is reassigned but name isn't.
3. Line 5839: Changed let { summarizeScores, comparisonExperimentId } destructuring to const with renaming (comparisonExperimentIdOpt), then added a let comparisonExperimentId since it's reassigned later.