Add Staging Extension To Steam Community Initiator Domains

[Perplex]

ref CSF-1080

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Note

Low Risk
Single JSON configuration change that only broadens an allowlist of extension initiator IDs; minimal functional surface area.

Overview
Updates src/steamcommunity_ruleset.json to add a second allowed initiatorDomains entry, enabling the existing modifyHeaders rule (setting referer for https://steamcommunity.com/tradeoffer/new/send XHRs) to apply when requests originate from the staging extension as well as the current one.

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