feat(rundown): edit multiple events at once in the entry editor

Selecting several events in the editor now turns the entry editor into a
multi editor: fields shared by every selected event show their value,
fields which differ show as mixed, and editing a field applies it to the
whole selection through the existing batch endpoint.

Rather than branching between a single and a multi editor, the editor now
always renders a merged view of N events (N >= 1) where a field is
undefined when the events disagree. For a single event nothing is ever
undefined, so single event editing is unchanged by construction and the
composites carry no multi edit branching.

- add mergeEvents() to build the merged view over a selection
- EventEditor takes an events array and resolves the write path itself,
  replacing the eventId threaded into the composites with a submit callback
- extract EventEditorSchedule, which is only rendered for a single event
  since schedule values cascade through the rundown
- support indeterminate values in Switch, Select, SwatchSelect and TimeInput
- cue, event id, schedule and automations are not available in multi edit

Two related fixes which multi edit depends on:
- useReactiveTextInput submitted an empty string when the initial value was
  undefined, so leaving a mixed field without editing it would have wiped
  the value on every selected event
- the batch optimistic update spread the request body onto the entry
  instead of the patch, and replaced custom fields rather than merging them

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PDETGBrhwgAqgTHjmrmJGJ

apps/client/src/features/rundown/entry-editor/RundownEntryEditor.tsx

@@ -1,4 +1,4 @@
-import { OntimeEntry, isOntimeEvent, isOntimeGroup, isOntimeMilestone } from 'ontime-types';
+import { OntimeEntry, OntimeEvent, isOntimeEvent, isOntimeGroup, isOntimeMilestone } from 'ontime-types';
 import { useMemo } from 'react';
 
 import useRundown from '../../../common/hooks-query/useRundown';
@@ -15,6 +15,25 @@ export default function RundownEntryEditor() {
   const selectedEvents = useEventSelection((state) => state.selectedEvents);
   const { data } = useRundown();
 
+  /**
+   * Events in the current selection
+   * Only events can be multi selected, groups and milestones are always selected on their own
+   */
+  const events = useMemo<OntimeEvent[]>(() => {
+    if (data.order.length === 0) {
+      return [];
+    }
+
+    const selection: OntimeEvent[] = [];
+    selectedEvents.forEach((entryId) => {
+      const entry = data.entries[entryId];
+      if (isOntimeEvent(entry)) {
+        selection.push(entry);
+      }
+    });
+    return selection;
+  }, [data.order.length, data.entries, selectedEvents]);
+
   const entry = useMemo<OntimeEntry | null>(() => {
     if (data.order.length === 0) {
       return null;
@@ -29,19 +48,20 @@ export default function RundownEntryEditor() {
     return event ?? null;
   }, [data.order.length, data.entries, selectedEvents]);
 
-  if (!entry) {
-    return <EventEditorEmpty />;
-  }
-
-  if (isOntimeEvent(entry)) {
+  if (events.length > 0) {
+    const singleEvent = events.length === 1 ? events[0] : null;
     return (
       <div className={style.rundownEditor} data-testid='editor-container'>
-        <EventEditor event={entry} />
-        <EventEditorFooter id={entry.id} cue={entry.cue} />
+        <EventEditor events={events} />
+        {singleEvent && <EventEditorFooter id={singleEvent.id} cue={singleEvent.cue} />}
       </div>
     );
   }
 
+  if (!entry) {
+    return <EventEditorEmpty />;
+  }
+
   if (isOntimeMilestone(entry)) {
     return (
       <div className={style.rundownEditor} data-testid='editor-container'>