feat(experimentation): add experiment creation wizard frontend

[Zaimwa9]

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Changes

Builds on #7591 — adds the frontend for experiment creation as a 4-step wizard accessible from the environment sidebar.

New components:

• ExperimentsPage — list/create mode toggle with empty state, experiment count, and "Create Experiment" CTA
• CreateExperimentWizard — orchestrates the 4-step wizard with state management and API submission
• WizardStepper — left sidebar step navigation with connecting lines, completion badges, and click-to-go-back
• WizardNavButtons — Back / Continue / Create Experiment navigation
• LivePreviewPanel — placeholder card (hidden on small breakpoints)
• ContentCard — reusable bordered card component (extracted for reuse across experimentation and warehouse UIs)
• VariationTable — read-only 3-column table (Name, Description, Value) matching the POC design

Steps:

• Step 1 (Setup) — fully functional: experiment name, hypothesis, searchable feature flag selector (multivariate only, client-side filtered), read-only variations table
• Step 2 (Audience & Traffic) — static placeholder: targeting card ("All identities") + sample size presets (100% selected)
• Step 3 (Measurement) — static placeholder: disabled search bar + disabled "Create Metric" button
• Step 4 (Review & Launch) — setup summary with Edit link, confirmation modal before creating

RTK Query service:

• useGetExperimentsQuery — list experiments
• useCreateExperimentMutation — create experiment (status=created)

Types:

• Experiment, ExperimentStatus in responses.ts
• getExperiments, createExperiment in requests.ts

Not in scope:

• Backend type filter on features endpoint (client-side MV filtering for now)
• Audience targeting conditions
• Metrics creation/selection
• Variation split configuration
• Live preview content
• Experiment detail/edit page
• Start/pause/complete actions

How did you test this code?

• Manual testing via ENV=local npm run dev with API running locally
• Navigated to Experiments page → verified empty state renders
• Clicked "Create Experiment" → verified wizard renders with all 4 steps
• Filled in Step 1 fields → verified validation (Continue disabled until all fields filled)
• Selected multivariate feature → verified variations table renders with POC styling
• Navigated through Steps 2-3 (placeholder pass-through) → verified Back/Continue work
• Verified Step 4 review summary shows correct data with Edit link
• Verified confirmation modal appears before creation
• Verified experiment created via POST to backend (after enabling experimental_flags flag and running migrations)
• Verified stepper shows completion state and allows click-back navigation

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Project	Deployment	Actions	Updated (UTC)
flagsmith-frontend-preview	Ready	Preview, Comment	May 28, 2026 1:57pm
flagsmith-frontend-staging	Ready	Preview, Comment	May 28, 2026 1:57pm

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	May 28, 2026 1:57pm

[Zaimwa9]

Thanks for the review, I have fixed all your comments. Concerning Storybook, noted, for now I have left them on the side but I will create the stories in the follow-up PR that uses backend filtering of MV

[talissoncosta]

Missed some on the previous round.
Can you please check that ? No blockers tbh..