* fix: auto-naming for 24h time format
needs_auto_name() required AM/PM suffix for default
frontend-generated names like 'deepseek-v4-flash 17:46:02'.
Frontend uses toLocaleTimeString() which outputs 24h
format in most locales — so the regex never matched and
auto-naming silently skipped.
Made AM/PM optional and added re.IGNORECASE for 'am'/'pm'.
* test: add regression tests for needs_auto_name (24h + 12h + custom)
---------
Co-authored-by: Calculator Dev <dev@calculator.local>
