Reduce schedule "Add" buttons to a single "+" icon

Task #190.

User wanted both add affordances in the Executive Meetings schedule
to show only the "+" symbol — no accompanying text in either
language.

Changes:
- artifacts/tx-os/src/pages/executive-meetings.tsx:
  - Add-row button: drops the visible label in idle state, renders
    only the <Plus> icon (aria-hidden). Loading state still shows
    "Loading..." for feedback. Added aria-label and aria-busy on
    the <button> so screen readers still announce the action and
    the loading state.
  - Add-attendee inline button: visible content is now the literal
    "+" character. aria-label preserved for assistive tech.
- artifacts/tx-os/src/locales/ar.json:
  - executiveMeetings.schedule.addRow: "أضف اجتماع جديد" → "أضف اجتماع"
  - executiveMeetings.schedule.addAttendee: "+ أضف حاضرًا" → "أضف حاضر"
- artifacts/tx-os/src/locales/en.json:
  - executiveMeetings.schedule.addAttendee: "+ Add attendee" → "Add attendee"

Both locale strings are now used solely as accessible names since
the visible glyph is hard-coded.

Verification: All 4 tests in
executive-meetings-edit-toggle.spec.mjs pass. Tests use
data-testid (em-add-row-button, em-add-attendee-*), not text, so
no test changes were required. Pre-existing TS errors in admin.tsx
and use-notifications-socket.ts are unrelated (api-client-react
codegen out of sync; tracked by other in-flight tasks).

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

@@ -854,13 +854,13 @@
     "dragRow": "اسحب لإعادة الترتيب",
     "editMeetingTitle": "تعديل عنوان الاجتماع",
     "schedule": {
-      "addRow": "أضف اجتماع جديد",
+      "addRow": "أضف اجتماع",
       "newMeetingDefaultAr": "اجتماع جديد",
       "newMeetingDefaultEn": "New meeting",
       "timeStart": "وقت البداية",
       "timeEnd": "وقت النهاية",
       "timeOrderError": "وقت النهاية يجب أن يكون بعد وقت البداية",
-      "addAttendee": "+ أضف حاضرًا",
+      "addAttendee": "أضف حاضر",
       "addVirtualAttendee": "+ حاضر عبر الاتصال المرئي",
       "addInternalAttendee": "+ حاضر داخلي",
       "addExternalAttendee": "+ حاضر خارجي",

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

@@ -809,7 +809,7 @@
       "timeStart": "Start time",
       "timeEnd": "End time",
       "timeOrderError": "End time must be after start time",
-      "addAttendee": "+ Add attendee",
+      "addAttendee": "Add attendee",
       "addVirtualAttendee": "+ Virtual",
       "addInternalAttendee": "+ Internal",
       "addExternalAttendee": "+ External",

artifacts/tx-os/src/pages/executive-meetings.tsx

@@ -1531,15 +1531,16 @@ function ScheduleSection({
                       type="button"
                       onClick={createRow}
                       disabled={creatingRow}
+                      aria-label={t("executiveMeetings.schedule.addRow")}
+                      aria-busy={creatingRow}
                       className="w-full flex items-center justify-center gap-2 py-2.5 text-sm text-[#0B1E3F] hover:bg-blue-50/60 focus:bg-blue-50/60 focus:outline-none transition-colors disabled:opacity-60"
                       data-testid="em-add-row-button"
                     >
-                      <Plus className="w-4 h-4" />
-                      <span>
-                        {creatingRow
-                          ? t("common.loading")
-                          : t("executiveMeetings.schedule.addRow")}
-                      </span>
+                      {creatingRow ? (
+                        <span>{t("common.loading")}</span>
+                      ) : (
+                        <Plus className="w-4 h-4" aria-hidden="true" />
+                      )}
                     </button>
                   </td>
                 </tr>
@@ -3007,7 +3008,7 @@ function AttendeeFlow({
             aria-label={addLabel}
             className="text-xs text-blue-600 hover:text-blue-700 hover:bg-blue-50 rounded px-2 py-1 min-h-[1.5rem] border border-dashed border-blue-300 hover:border-blue-500 transition-colors"
           >
-            {addLabel}
+            +
           </button>
         </li>
       )}