Fix stratifications table not loading on page refresh

[zackcl]

Resolves #2930

Problem

Stratifications table was empty after refreshing the Participants page while the Preview Users tab is temporarily disabled.

Solution

• Changed ngOnInit to call selectedTabChange({index: 1}) to match current tab structure
• Added sequential data loading: fetch experiment names first, then stratifications after names are loaded (otherwise, an error occurs) using filter and take(1)

Changes

• experiment-users-root.component.ts: Sequential fetch logic for stratifications tab
• Child component remains unchanged (subscribe-only pattern)

Revert Instructions

When re-enabling Preview Users tab:

1. Uncomment tab in HTML
2. Change index: 1 back to index: 0 in ngOnInit

[Copilot]

Pull request overview

This PR fixes a bug where the stratifications table would not load when refreshing the Participants page while the Preview Users tab is temporarily disabled.

Changes:

• Updated tab initialization to match current tab structure (index 1 instead of 0)
• Implemented sequential data loading to ensure experiment names are fetched before stratifications

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