Task #171: Schedule Edit/View toggle (final fixes)

Add a single global "تحرير/Edit" toggle button to the schedule
toolbar that hides every editing affordance by default and reveals
them only when the user (with edit permission) explicitly opts in.

Affordances now gated behind `effectiveCanMutate = canMutate &&
editMode`:
- "+ Add row" button
- Per-row delete, color swatch, merge trigger, drag grip
- Inline cell editors (EditableCell, TimeRangeCell)
- Column drag-reorder (SortableHeader.dragEnabled)
- Column resize handles
- "+ Add attendee" button AND its pending ghost row

Persistence:
- Toggle state is stored in localStorage under a per-user key
  `em-schedule-edit-mode-v1:<userId>`, so a shared browser cannot
  leak one editor's last toggle into another account that signs
  in. Falls back to view mode when userId is unavailable.
- Always starts in view mode for users without edit permission.

Toggle-off safety:
- EditableCell + TimeRangeCell discard any in-progress draft and
  exit edit mode when their `disabled` / `canMutate` prop flips.
- ScheduleSection clears `pendingAttendee` in a useEffect when
  effectiveCanMutate becomes false, so the ghost "+ Add attendee"
  row unmounts immediately.
- AttendeeFlow also gates the pending render on `canMutate` as
  defense in depth.

i18n: 4 new keys under `executiveMeetings.schedule`
(editToggle / editToggleAria / editToggleOn / editToggleOff)
in both en.json and ar.json.

Tests: tests/executive-meetings-edit-toggle.spec.mjs covers
default-hidden affordances, toggle-on reveal, reload persistence,
and toggle-off re-hide. Cleanup wipes all `em-schedule-edit-mode-v1*`
keys to handle the user-namespaced storage. Full e2e suite (12
tests) passes.

Code review: PASS on the second pass after the per-user key + ghost
row fixes. Pre-existing TS errors in admin.tsx and
use-notifications-socket.ts are codegen drift from earlier tasks
and are not touched by this change.

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

@@ -828,7 +828,11 @@
       "editAttendeeAria": "تعديل اسم الحاضر",
       "deleteRow": "حذف الاجتماع",
       "deleteRowConfirm": "حذف هذا الاجتماع؟\n\n\"{{title}}\"\n\nلا يمكن التراجع عن هذا الإجراء.",
-      "deleted": "تم حذف الاجتماع"
+      "deleted": "تم حذف الاجتماع",
+      "editToggle": "تحرير",
+      "editToggleAria": "تبديل وضع التحرير",
+      "editToggleOn": "وضع التحرير مفعّل",
+      "editToggleOff": "وضع التحرير معطّل"
     },
     "rowColor": {
       "label": "لون الصف"