fix(flow-chat): center session header titles - #1771

Summary

• center SceneBar tab content against the full tab width with symmetric close-button reservation
• center the active turn title against the full conversation header instead of the remaining flex space
• keep the centered safe area responsive to search and action-group width changes
• add regression coverage for asymmetric header action widths

Root cause

The tab content reserved space only on the right, while the turn title was centered inside the space left between unequal action groups. Both produced a visible left offset from the conversation surface center.

Verification

• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/FlowChatHeader.test.tsx
• pnpm run type-check:web
• focused ESLint check for FlowChatHeader.tsx
• local Vite render: SceneBar content center offset measured at 0px