fix: stop settings.json from being wiped on launch

Electron main, ssh-manager, and the embedded web server all write the
same settings.json. readJsonFile/readJsonRoot silently coerced any read
failure (including mid-write parse errors) to {}, and writes were plain
fs.writeFile. A partial read during a concurrent write let the reader's
next read-modify-write overwrite the whole file with only the field it
just set — wiping projects, desktopDefaultHostId, and more. Next launch
showed the welcome chooser because defaultHostId was gone, and the
sidebar was empty because projects were gone.

- Switch all writers to atomic tmp+rename so readers never see partial
  JSON.
- Add mutateSettingsRoot() in Electron main to serialize read-modify-
  write pairs across its own call sites (hosts config, window state,
  desktop port, ssh instances, vibrancy).
- Keep read-on-error returning {} to avoid crashing startup callers,
  but log loudly now so we can catch it if it ever happens again.
- useProjectsStore: don't clobber a populated cache with empty incoming
  settings. If settings ever do come back empty, the sidebar stays
  intact until a real, non-empty sync lands.

packages/ui/src/stores/useProjectsStore.ts

@@ -655,6 +655,25 @@ export const useProjectsStore = create<ProjectsStore>()(
         : null;
 
       const current = get();
+
+      // Race guard: settings load can return empty projects during app
+      // rebuild/reinstall or an incomplete settings read. Don't clobber
+      // a populated cache with empty — the sidebar would go blank and
+      // localStorage would be overwritten, losing the list entirely.
+      if (incomingProjects.length === 0 && current.projects.length > 0) {
+        if (incomingActive !== current.activeProjectId) {
+          // Active project may still be valid within the cached list.
+          const activeExists = incomingActive
+            ? current.projects.some((project) => project.id === incomingActive)
+            : true;
+          if (activeExists) {
+            set({ activeProjectId: incomingActive });
+            cacheProjects(current.projects, incomingActive);
+          }
+        }
+        return;
+      }
+
       const projectsChanged = JSON.stringify(current.projects) !== JSON.stringify(incomingProjects);
       const activeChanged = current.activeProjectId !== incomingActive;