An update that goes wrong is no longer the end of it

Two things the audit found, both about the archive path every shipped .app
uses.

**Settings could be wiped.** The update kept config/modules.json and
config/layouts/ by name, but not config/proddash.json — which holds the admin
PIN, the theme and the forge pointer. On an install whose data directory IS
the app's own config/ (an unwritable Application Support, or PRODDASH_DATA_DIR
aimed inside the app), an update wrote the repo's shipped defaults over it:
the theme went, and admin silently unlocked. Nothing inside the data
directory is written now, wherever it happens to sit, and the two legacy
names stay listed for an install whose state never moved out.

**A bad update was unrecoverable.** No rollback, and a fresh .app would not
help: seeding only replaces an *older* copy, and a broken one carries a
higher version — so the fix was deleting a folder nothing told you about.
Three changes, in the order they catch things:

- The download must parse before anything is swapped in. `node --check`
  against the new server.js, and a failure is reported with the syntax error
  and leaves the running copy untouched.
- Past that, the copy being replaced is set aside as `app.previous`, and
  dropped once the new version has stayed up ninety seconds — so that folder
  existing means "the last update has not proven itself yet", and never
  anything older.
- The launcher, which already gives up after four stops in two minutes, now
  looks for that folder before it does and puts the old copy back. Once: if
  the old one will not run either, the update was not the problem.

The launcher half only reaches a machine with a new .app; the server half
travels by the normal update.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

launcher/macos/Sources/LauncherConfig.swift

@@ -103,6 +103,45 @@ enum ProdDash {
         }
     }
 
+    /**
+     Go back to the copy ProdDash set aside before its last update.
+
+     A version that will not stay up is the one state this app cannot talk its
+     way out of: the admin page went down with it, and installing a fresh
+     `.app` does nothing, because seeding only replaces an *older* copy and a
+     broken one carries a higher version. So ProdDash writes `app.previous`
+     before it overwrites itself and drops it again once the new version has
+     stayed up — meaning that folder exists only while an update is still
+     unproven. This puts it back.
+
+     Returns the version restored, or nil when there is nothing to go back to.
+     */
+    @discardableResult
+    static func restorePrevious(root: URL, log: (String) -> Void) -> String? {
+        let fm = FileManager.default
+        let previous = URL(fileURLWithPath: root.path + ".previous")
+        guard isRoot(previous) else { return nil }
+        let version = shellVersion(root: previous)
+        let named = version.isEmpty ? "(unknown version)" : version
+        let broken = URL(fileURLWithPath: root.path + ".broken")
+        do {
+            try? fm.removeItem(at: broken)
+            if fm.fileExists(atPath: root.path) { try fm.moveItem(at: root, to: broken) }
+            try fm.moveItem(at: previous, to: root)
+            makeWritable(root)
+            try? fm.removeItem(at: broken)
+            log("rolled ProdDash back to \(named) — the copy it replaced")
+            return named
+        } catch {
+            log("could not roll back to the previous copy: \(error.localizedDescription)")
+            // Leave something runnable behind rather than nothing at all.
+            if !fm.fileExists(atPath: root.path), fm.fileExists(atPath: broken.path) {
+                try? fm.moveItem(at: broken, to: root)
+            }
+            return nil
+        }
+    }
+
     /// Files copied out of a bundle can arrive read-only, and this copy is
     /// exactly where ProdDash writes its own updates.
     private static func makeWritable(_ root: URL) {