Centralize UI session state and show thread activity

[VolkerChristian]

Summary

• centralize retained frontend state and toolkit-neutral view projection in UiSession
• keep native dialogs, prompt admission, request policy, conversation projection, and inspectors on the shared session boundary
• show Last activity in the Conversation header for native CodexUI and CodexWUI
• initialize activity from max(updatedAt, recencyAt) and advance it monotonically for meaningful live thread-scoped traffic
• preserve authoritative activity during selection-driven thread/read and thread/resume hydration
• right-align the timestamp while preserving the title/workspace/state baseline and responsive WebUI layout
• document the activity semantics and expanded UI-session architecture

Verification

• cmake --build /home/voc/projects/drafts/CodexUI/build/Desktop_GCC-Debug -j2
• ctest --test-dir /home/voc/projects/drafts/CodexUI/build/Desktop_GCC-Debug --output-on-failure (9/9)
• npm test in web/ (76/76)
• git diff --check