feat(perf): explicit toggle for live thread-group CPU polling (#891)

* feat(perf): explicit toggle for live thread-group CPU polling

Replace implicit thread-group collection (section open / pin) with a persisted
checkbox so Linux /proc scans run only when the user opts in for diagnosis.

Fix IPC: pass includeThreadGroups (camelCase) so Tauri maps the flag to Rust;
reset the CPU baseline when the option changes so thread % deltas are valid.

* docs: CHANGELOG and credits for PR #891

* fix(perf): gate CHILD_RESCAN_EVERY to Linux/macOS only

Avoid dead_code warning on Windows where perf child-PID rescan is unused.

src/utils/perf/perfLiveStore.ts

@@ -231,6 +231,8 @@ export function bumpPerfLivePollSchedule(): void {
     window.clearTimeout(pollTimer);
     pollTimer = null;
   }
+  // Fresh baseline after interval / thread-group option changes.
+  prevProc = null;
   schedulePoll();
 }