fix(chat): stop active runs and background processes immediately

[24baigei]

Summary

• Make Stop update GUI and Gateway WebUI state immediately instead of waiting for the active turn to unwind.
• Bind abort controllers, runtime state, transcript/tool status, compaction state, snapshots, and timers to the exact run so stale finalizers cannot overwrite a newer turn.
• Prevent stopped turns from advancing the queue or being revived by late cancelling, hydration, or replay events.
• Propagate the user-stop signal across parent/child agents and terminate non-isolated managed process trees.
• Preserve existing isolated=true long-running process behavior.

Cross-platform behavior

• macOS/Linux: terminate the managed Unix process group with TERM, followed by KILL when needed.
• Windows: terminate the full process tree with taskkill /T /F.
• GUI and Gateway WebUI share the same immediate-stop and stale-run protections.
• The Unix process-group path could not be exercised on the Windows development host and should be confirmed by macOS/Linux CI.

Verification

• 36/36 targeted GUI tests passed.
• 120/120 targeted Gateway/WebUI tests passed.
• GUI and WebUI TypeScript checks passed.
• GUI and WebUI production builds passed.
• git diff --check passed.
• Biome lint passed with formatter disabled; only pre-existing repository warnings were reported.