chore: Wire feature flags to UI (SettingsPage, TestTab, MemoryTab)

[VictorGjn]

Context

Feature flags store (featureFlags.ts) and components (ExperimentalBadge, ExperimentalGate) were created in #136 but not yet integrated into the UI.

Changes needed

1. SettingsPage: Add "Experimental Features" section with toggles for each flag
2. TestTab: Wrap Team Runner section with ExperimentalGate feature="teamRunner"
3. MemoryTab: Non-functional backends (Redis/ChromaDB/Pinecone/Custom) disabled with ExperimentalBadge feature="advancedMemoryBackends" disabled
4. ToolsTab: Skills marketplace section gated behind skillsMarketplace flag

Depends on: #136 merge