Log filter: match case-insensitively

The Log window's filter used a case-sensitive substring match, so "error" missed
"ERROR" -- inconsistent with the rest of the app. Match case-insensitively.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -117,7 +117,6 @@ UX:
 - **Auto re-attach control** — keep it visible/controllable when not connected (today it can be
   enabled-but-invisible).
 - **Tooltips on disabled controls** — explain *why* Inject/subsystem controls are disabled.
-- **Case-insensitive log filter.**
 
 Code quality:
 - **Stale comments** — `hook_guard.hpp` top block (still the old destroy-on-remove model),