fix(hub): self-heal truncated/corrupt Nix downloads and keep failed updates retryable

Updater/rebuild self-heal:
- Add a shared run_step wrapper used by both the update and rebuild
  scripts. On the first failure matching a transient fetch/cache signature
  (truncated tarball, corrupt NAR, hash mismatch, network timeout,
  interrupted download), clear Nix's fetch caches and repair the store,
  then retry once. Real config errors do not match and still fail loudly.
- The kernel-change boot fallback in the rebuild path is also wrapped.
- Fixes the reported 'cannot read file from tarball: Truncated tar archive
  detected' failure, which a plain re-run cannot clear because Nix reuses
  the corrupt cached archive.

Failed-update recovery / reporting:
- check_for_updates() now compares the running Hub version against the
  branch VERSION, so a failed 'nix flake update' (lock advanced but no
  generation staged) can no longer masquerade as 'up to date' and block
  retries.
- /api/updates/check surfaces a persistent 'failed' state; /api/updates/run
  never blocks a retry after a failure.
- Dashboard shows a red 'Update failed - click to retry' tile; the modal
  offers a Retry Update button and stops offering a reboot on failure.

app/sovran_systemsos_web/templates/index.html

@@ -75,6 +75,7 @@
       <div class="modal-footer">
         <button class="btn btn-save" id="btn-save-report" style="display:none">Save Error Report</button>
         <button class="btn btn-save" id="btn-retry-update-status" style="display:none">Retry Status</button>
+        <button class="btn btn-reboot" id="btn-retry-update" style="display:none">Retry Update</button>
         <button class="btn btn-reboot" id="btn-reboot" style="display:none">Restart Entire System</button>
         <button class="btn btn-close-modal" id="btn-close-modal" disabled>Close</button>
       </div>