fix(tabs): report an edit dropped behind an in-flight save, respect the group cap (#525 landing)

- createEditCoordinator's finally block rebases the edits queued during a write; one that the write's 409 made inapplicable was dropped with no toast. It is now reported once, like the main loop and adoptExternal do (found by the PR bot's re-review; regression test fails without it).
- At the 32-group server cap the row and group menus no longer offer a new group, which could only fail with an untranslated 'group limit reached'. MAX_GROUPS is exported from tab-layout-browser.js.
- CLAUDE.md names the pagehide keepalive as the one deliberate exception to 'never PUT the layout outside the coordinator'.
- The Dashboard wiki page describes tab groups in the vertical rail row.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/tab-layout-browser.js

@@ -658,6 +658,11 @@
         writing = false;
         if (!disposed) {
           const rebased = replayOperations(authoritative, pending);
+          // Edits made while the write was in flight are rebased here, so one the
+          // conflict made inapplicable is dropped here too, and says so (once).
+          if (rebased.dropped && !failed && !reportedDrop) {
+            report('Tab groups changed elsewhere; part of your edit no longer applies.');
+          }
           pending = rebased.kept;
           optimistic = rebased.layout;
           publish({ authoritative: true });
@@ -740,5 +745,6 @@
     createLoadCoordinator,
     loadCollapsedGroupIds,
     saveCollapsedGroupIds,
+    MAX_GROUPS,
   };
 })(typeof window !== 'undefined' ? window : globalThis);