Three redundant store fields encoding the same AM/PM choice replaced with a single
`use_12h: boolean` in PresMgmtLocState. iso_datetime_formatter gains a third param
(use_12h: boolean | null = null) that auto-resolves 24h↔12h format name variants via
a symmetric FORMAT_PAIRS lookup — null default leaves all ~100 existing call sites intact.
Toggle surfaces in three places: Clock icon in session time chip (hidden button, same
visual), event Options modal Display section, and session Options modal Display section.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
631a77158c