Merge branch 'feat/2.7-common' into feat/2.7-ui

First sync down from the trunk under the agreed flow: common syncs into ui
whenever the trunk moves, and ui delivers back at milestones — always after
a sync, so the delivery merge carries no conflicts of its own.

Only two files conflicted, both trivial, which confirms the two branches
resolved the shared 2.6-vs-mainline hunks the same way:

- PlusMenu.tsx: kept the UI branch's semantic color tokens over the trunk's
  hardcoded hex; the tokens travel back to common on the next delivery.
- AiMessageBubble.tsx: identical code, comment wrapping only.

Both apps lint clean; platform typecheck clean; i18n parity clean; the six
remaining client typecheck errors are byte-identical to main.

src/frontend/client/eslint-suppressions.json

@@ -3076,7 +3076,7 @@
   },
   "src/pages/appChat/useChatHelpers.ts": {
     "@typescript-eslint/no-explicit-any": {
-      "count": 11
+      "count": 12
     },
     "@typescript-eslint/no-unused-vars": {
       "count": 2