chore: Feature flags for experimental features

[VictorGjn]

Problem

Team Runner, Contrastive Retrieval, Cost Intelligence, Analytics, Memory backends (Redis/ChromaDB/Pinecone) are present in the UI but not stable or functional. Creates confusion and dead-ends.

Success criteria

• src/store/featureFlags.ts with persisted flag system (localStorage)
• Non-stable features show "Experimental" badge + tooltip
• Settings page: toggle to enable/disable experimental features
• Memory tab: non-functional backends non-selectable with "Coming soon" tooltip

Implementation

1. Create src/store/featureFlags.ts (Zustand, localStorage)
2. Flags: teamRunner, contrastiveRetrieval, costIntelligence, analytics, advancedMemoryBackends
3. src/components/ds/ExperimentalBadge.tsx — reusable badge + tooltip
4. Wrap affected features: if flag off → show placeholder
5. Settings page: "Experimental Features" section with toggles