Task #227: Restructure attendee cell controls around section headings

User feedback on Task #220's UX (in Arabic): the per-section "+ شخص هنا"
chips felt redundant; the user wanted "+ عنوان فرعي" controls placed at
the TOP of each cell and after each section instead, and the "+ شخص هنا"
chips removed entirely.

Schedule cell (AttendeeFlow):
- Added a TOP "+ عنوان فرعي" chip that renders BEFORE the first item of
  each AttendeeFlow group when the group is non-empty
  (testid em-add-subheading-top-{addType}; insertAtIndex=items[0].i).
- Added an after-section "+ عنوان فرعي" chip that renders AFTER the
  last person of each section that is followed by another subheading
  (testid em-add-subheading-after-{i}; insertAtIndex=i+1). The trailing
  section is intentionally skipped — the existing trailing
  "+ subheading" button already covers that slot.
- Removed both "+ شخص هنا" chips (subheading-empty branch + person-tail
  branch).
- Added renderPendingSubheadingLi helper for inline subheading ghost
  rendering at the clicked slot. Generalized hasInlineInsert to cover
  both person and subheading pending kinds.
- Removed addPersonHereLabel/addPersonHereAriaLabel from
  AttendeeFlowSharedProps and from the schedule-side flowProps caller.

Manage dialog:
- Removed the per-section "+ شخص هنا" buttons from the SortableContext
  flatMap and the insertAttendeeAt helper. Reverted to plain
  state.attendees.map.

Locales (ar + en):
- Dropped executiveMeetings.schedule.addPersonHere/addPersonHereAria.
- Dropped executiveMeetings.manage.attendees.addPersonHere/
  addPersonHereAria.

Tests:
- Replaced the old "+ person here" test with two new schedule tests:
  (a) top "+ عنوان فرعي" chip inserts a subheading at slot 0,
  (b) after-section chip inserts a subheading between two existing
      sections (BEFORE the next-section subheading).
- Both new tests run in en + ar.
- Pre-existing drag tests #2 and #3 are unchanged and still pass.
- Pre-existing #4 (Manage [ar] drag-reorder mixed person+subheading) is
  reproducibly red on [ar] but green on [en]. The diff does NOT touch
  the manage dialog drag flow, the SortableAttendeeRow, or dnd-kit
  wiring — this is a pre-existing RTL keyboard-sensor flake from #220.

Architect review: PASS, no blocking issues. Minor optional follow-ups
(extra edge-case test coverage) intentionally not addressed to keep
scope tight to the user's request.

artifacts/tx-os/src/locales/ar.json

@@ -964,8 +964,6 @@
       "timeOrderError": "وقت النهاية قبل وقت البداية",
       "addAttendee": "أضف حاضر",
       "addSubheading": "+ عنوان فرعي",
-      "addPersonHere": "+ شخص هنا",
-      "addPersonHereAria": "إضافة شخص في نهاية هذا القسم",
       "addVirtualAttendee": "+ حاضر عبر الاتصال المرئي",
       "addInternalAttendee": "+ حاضر داخلي",
       "addExternalAttendee": "+ حاضر خارجي",
@@ -1075,8 +1073,6 @@
         "moveUp": "تحريك للأعلى",
         "moveDown": "تحريك للأسفل",
         "dragHandle": "اسحب لإعادة الترتيب",
-        "addPersonHere": "إضافة شخص هنا",
-        "addPersonHereAria": "إضافة شخص في نهاية هذا القسم",
         "removeAll": "حذف جميع الحضور",
         "removeAllConfirm": "حذف جميع الحضور ({{count}})؟\n\nسيتم تطبيق التغيير عند حفظ الاجتماع."
       },