fix(contests): show skeletons only until feature flag resolves

[dylanjeffers]

Summary

• Contests page disabled its useAllRemixContests query while useFeatureFlag(CONTESTS).isEnabled was still undefined during Optimizely's ~14s cold-load. Disabled queries report isPending: true, so the skeleton state stayed on screen indefinitely on first paint.
• Fix: start the query optimistically before the flag resolves by gating enabled on !isFlagLoaded || isContestsPageEnabled. The existing <Navigate to='/' /> continues to redirect once the flag loads as false, so users without access still can't see contest data.

Test plan

• Cold-load /contests with Optimizely throttled — verify contest cards render as soon as the API responds rather than waiting on the flag.
• Toggle the CONTESTS flag off — verify the page still redirects to / after the flag resolves.
• Flag on, no contests in response — verify empty state still renders correctly.

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[dylanjeffers]

Closing as obsolete: main dropped the CONTESTS feature flag in #14317, so there is no flag-resolution gating left for skeletons to wait on. The skeletons-on-load behavior collapses to the simple 'show until isPending flips' case already in the component.