fix(desktop): clear stale session bubbles and busy state on new chat

[Morad37]

After #37975, a newly-started chat could visually show stale user bubbles and red session busy errors from another live session. This was render contamination, not model-context contamination — the assistant in the new chat ignored the stale visuals.

Two changes:

1. startFreshSessionDraft now clears workingSessionIds and attentionSessionIds, so background sessions that were still working when navigated away from don't persist as running rows in the new chat.
2. Added a guard in the session cache update to only update global working/attention atoms when the session is the currently-viewed one. Background sessions update their own cache entry but can't re-add themselves to the sidebar indicators after being cleared.

Fixes #39086

[teknium1]

Thanks for isolating the new-chat/session-state interaction. The transcript-bleed portion is already guarded on current main, but the proposed status handling conflicts with the current background-session model.

Problems

• use-session-state-cache.ts on current main intentionally records working and attention state for every session at apps/desktop/src/app/session/hooks/use-session-state-cache.ts:263-271; sidebar rows consume those atoms at apps/desktop/src/app/chat/sidebar/index.tsx:252,342 and apps/desktop/src/app/chat/sidebar/session-row.tsx:80.
• The active-session-only update proposed here would therefore hide legitimate background work and clarify prompts. Commit 3c163cb0353c6dc5e0b5278998e721368ed71a68 introduced the persistent background needs-input indicator for that purpose.
• Current foreground isolation is already enforced at apps/desktop/src/app/session/hooks/use-session-state-cache.ts:176-189, which rejects background state before it reaches the shared view.

Suggested changes

• Re-scope around a reproducible stale-ID lifecycle issue, preserving indicators for genuinely live background sessions.
• Add a regression covering a fresh draft while a separate session remains busy or needs input.

Automated hermes-sweeper review.