feat: tag queue new-feature-badge variation as Amplitude user property

[dylanjeffers]

Summary

Sets an Amplitude user property queue_new_feature_badge = 'on' | 'off' so the A/B test for the play-queue "New" badge (#14254) can be sliced in Amplitude. Currently nothing surfaces the variation to Amplitude, so charts can't filter by treatment vs. control.

• Adds queue_new_feature_badge?: 'on' | 'off' to IdentifyTraits.
• In useQueueNewFeatureBadge, dispatches ANALYTICS/IDENTIFY once when the feature flag has loaded, with the variation as a trait. The existing analytics saga forwards it to both web and mobile Amplitude SDKs as a user property via Identify.set(...).
• Per-mount useRef guard so it dispatches at most once per mount; Amplitude Identify.set is idempotent so re-fires across mounts are harmless.

Optimizely setup (separate, not in this PR)

Create a flag named queue_new_feature_badge (lowercase, exact) with a Targeted Delivery rule at 50% included → variation on. The flag default is false (feature-flags.ts), so until the rule is on, all users get tagged 'off'.

Test plan

• In dev with the flag forced on, confirm an Identify call fires once with queue_new_feature_badge: 'on' (Amplitude network panel / Redux DevTools ANALYTICS/IDENTIFY action).
• In dev with the flag forced off, same check with 'off'.
• Verify on web (QueueButton) and mobile (ActionsBar) — both consume the hook.
• Verify in Amplitude that the user property shows up on the next event after the dispatch.

🤖 Generated with Claude Code

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