feat(pres_mgmt): replace time_hours/time_format/datetime_format with single use_12h toggle

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>

src/lib/stores/ae_events_stores__pres_mgmt_defaults.ts

@@ -84,9 +84,7 @@ export interface PresMgmtLocState {
     lock_config: boolean;
 
     // --- Query / search preferences ---
-    datetime_format: string;
-    time_format: string;
-    time_hours: 12 | 24;
+    use_12h: boolean;
     qry_enabled: 'all' | 'not_enabled' | 'enabled';
     qry_hidden: 'all' | 'hidden' | 'not_hidden';
     qry_limit__files: number;
@@ -265,9 +263,7 @@ export const pres_mgmt_loc_defaults: PresMgmtLocState = {
     lock_config: false,
 
     // Query / search
-    datetime_format: 'datetime_12_long',
-    time_format: 'time_12_short',
-    time_hours: 12,
+    use_12h: true,
     qry_enabled: 'enabled',
     qry_hidden: 'not_hidden',
     qry_limit__files: 75,