refactor(chat): decouple visible session from live socket runtime in AgentDetail

[Congregalis]

Summary

This PR is a follow-up to the session-switching bugfix work (#144 / #151).
It separates UI visibility from per-session chat runtime in AgentDetail.tsx, so switching sessions/agents no longer relies on a single global socket lifecycle.

Problem

AgentDetail previously treated:

• the currently visible session
• and the currently running conversation socket

as the same lifecycle.

That coupling made session switching fragile and caused issues such as:

• stale responses leaking across sessions,
• inconsistent streaming/waiting behavior on session transitions,
• race conditions when rapidly switching sessions.

What changed

1) Per-session runtime management

• Introduced runtime keying by agentId:sessionId.
• Replaced single-socket assumptions with per-session socket/runtime tracking.

2) Session message load race protection

• Added AbortController + sequence checks in selectSession().
• Prevents earlier (stale) session-message requests from overwriting newer selection state.

3) Unified session-switch behavior

• createNewSession() now reuses session selection flow instead of ad-hoc state resets.
• Session deletion now also cleans corresponding runtime/socket state.

4) Active-session scoped send/stop actions

• Send and abort now target the active session runtime socket explicitly.
• Avoids sending/stopping on the wrong socket.

5) Preserve “thinking/waiting” state across switches

• Added per-session UI state cache for isWaiting / isStreaming.
• If user switches away and returns while response is still pending, the “thinking” indicator is restored.

Scope / Non-goals

This PR focuses on stabilizing in-page session/agent switching behavior in AgentDetail.
It does not introduce a full background execution/job model across page reloads.

Validation

Manual checks performed:

1. Session A streaming -> switch to session/agent B -> switch back to A: messages remain intact.
2. If A is still pending, returning to A restores waiting/streaming visual state.
3. Rapid session switches do not allow stale message fetch responses to overwrite current selection.

Files changed

• frontend/src/pages/AgentDetail.tsx

Checklist

• Tested locally
• No unrelated changes included