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/components/Sidebar.tsx

@@ -368,15 +368,25 @@ export function Sidebar({ wsConnected, collapsed, onToggle }: SidebarProps) {
     ? t("nav:checkingForUpdates")
     : checkError
       ? t("nav:checkFailed")
-      : updateAvailable
-        ? updateStatus?.latest_release
-          ? t("nav:releaseAvailable", { version: updateStatus.latest_release })
-          : t("nav:updateAvailable")
-        : updateStatus?.dev_build
-          ? t("nav:devBuild", { count: updateStatus.commits_ahead_of_release ?? 0 })
-          : updateStatus
-            ? t("nav:upToDate")
-            : t("nav:checkForUpdates");
+      : updateStatus?.installed_not_running
+        ? t("nav:installedNotRunning", {
+            version: updateStatus.installed_not_running.version ?? "?",
+          })
+        : updateAvailable
+          ? updateStatus?.latest_release
+            ? t("nav:releaseAvailable", { version: updateStatus.latest_release })
+            : t("nav:updateAvailable")
+          : updateStatus?.dev_build
+            ? t("nav:devBuild", { count: updateStatus.commits_ahead_of_release ?? 0 })
+            : // A plugin install has no commit count to fall back on: a release
+              // that could not be read is "unknown", never "up to date".
+              updateStatus?.install_kind === "plugin" &&
+                updateStatus.release_check &&
+                updateStatus.release_check !== "ok"
+              ? t("nav:releaseUnchecked")
+              : updateStatus
+                ? t("nav:upToDate")
+                : t("nav:checkForUpdates");
   const currentLanguage = normalizeLanguage(i18n.resolvedLanguage ?? i18n.language);
   const currentIndex = SUPPORTED_LANGUAGES.indexOf(currentLanguage);
   const nextLanguage = SUPPORTED_LANGUAGES[(currentIndex + 1) % SUPPORTED_LANGUAGES.length];