Task #152: cell-merge + current-meeting tint on EM schedule

Adds two related features to the executive-meetings schedule:

1. Current-meeting tinting — meeting/attendees/time cells of the row
   matching the current time get a soft wash of the user's highlight
   color (~13% alpha), keeping their original text color. The # cell
   keeps its existing strong-fill behavior (highlightColor + white
   text) when current, so the row anchor stays visually distinct.

2. Cell-merge overlay — editors can merge "meeting+attendees",
   "meeting+attendees+time", or the whole row into a single free-text
   cell. Stored in the DB so all viewers see the same overlay; the
   originals (titleAr/En, attendees, start/endTime) stay untouched
   and are restored on Unmerge.

Implementation notes
- DB: 3 nullable columns added to executive_meetings
  (merge_start_column, merge_end_column, merge_text). Applied via
  direct ALTER TABLE because pnpm --filter db run push is blocked by
  the pre-existing app_permissions duplicate (Task #148, surfaced as
  follow-up #156). Schema file kept in sync.
- API: meetingPatchSchema accepts an optional `merge` field
  (`null | {start,end,text}`) with start<=end ordering and server-side
  sanitizeRichText on the text. Reuses the existing PATCH route.
- Frontend: new MergeMenu component + canonical-order range
  resolution so merges still render when boundary columns are hidden,
  and gracefully degrade to unmerged rendering (without losing DB
  state) when column reordering makes the visible span non-contiguous.
  Unmerge stays available whenever a stored merge exists, even in
  degraded state, and existing mergeText is preserved on re-merge.
  Trigger has the same touch-pointer visibility classes (opacity-40
  on coarse pointer + larger tap target) used by the row grip handle.
- i18n: en + ar keys under executiveMeetings.merge.*
- Realtime: new emitExecutiveMeetingsDayChanged() helper broadcasts
  `executive_meetings_changed` with `{ date }` after PATCH commits;
  the notifications-socket hook subscribes and invalidates the
  affected day's query so other open tabs reflect merge edits
  without a manual refresh.

Out of scope (deferred to follow-ups)
- API + e2e test coverage for the merge endpoint and UI flows (#154)
- Realtime emission for the remaining EM mutations — POST/DELETE/
  PUT/duplicate/reorder (#155)

Validation
- TypeScript compiles cleanly across the changed surfaces (existing
  pre-existing errors in api-zod / generated client are unrelated).
- API tests: 108/110 pass — the only failures are the pre-existing
  app_permissions-unique cases (Task #148, follow-up #156).
- Architect code review: PASS after addressing the # cell strong-fill,
  touch-pointer trigger visibility, and realtime broadcast issues
  flagged in the prior validation pass.

artifacts/tx-os/src/hooks/use-notifications-socket.ts

@@ -71,6 +71,21 @@ export function useNotificationsSocket() {
       queryClient.invalidateQueries({ queryKey: getGetMeQueryKey() });
     });
 
+    socket.on(
+      "executive_meetings_changed",
+      (payload?: { date?: string }) => {
+        if (typeof payload?.date === "string" && payload.date.length > 0) {
+          queryClient.invalidateQueries({
+            queryKey: ["/api/executive-meetings", payload.date],
+          });
+        } else {
+          queryClient.invalidateQueries({
+            queryKey: ["/api/executive-meetings"],
+          });
+        }
+      },
+    );
+
     socket.on(
       "role_permissions_changed",
       (payload?: { roleId?: number }) => {