fix(updates): report a plugin build installed beside the running one, never "up to date" on a failed check

A `claude plugin update` run from a terminal (or with only the Claude desktop
app open) unpacks the new build and starts nothing: no SessionStart fires, so
the dashboard kept running the old cache dir across reboots, and with the
forge unreachable the sidebar read "Up to date" (seen on a machine stuck at
1.8.10).

- pluginStatus asks supersededBy for its own root: a recorded sibling install
  sets `installed_not_running: {root, version}` and `update_available`, with
  no network needed
- the sidebar reads "<version> installed, not running"; the update modal keys
  on it, so Update now is offered even with no release tag or sha
- plugin-update-helper hands over to the recorded install first, the newest
  sibling dir only as a fallback
- a plugin install whose release lookup failed reads "Could not check for
  updates" instead of "Up to date"
- docs/PLUGINS.md: the case and the manual hand-over command

client/src/i18n/locales/vi/nav.json

@@ -68,5 +68,7 @@
   "workspace": "Không gian làm việc",
   "wsEndpoint": "WebSocket",
   "releaseAvailable": "Có {{version}}",
-  "devBuild": "Bản dev · trước {{count}} commit"
+  "devBuild": "Bản dev · trước {{count}} commit",
+  "installedNotRunning": "Đã cài {{version}}, chưa chạy",
+  "releaseUnchecked": "Không kiểm tra được bản mới"
 }