gui: harden the restart relaunch (ultra edge cases)

Follow-up to the Restart Now button. Addresses the edge cases the review
flagged, and drops the fragile batch layer entirely:

- Windows no longer writes a .bat and shells through cmd (which re-parsed
  the arguments and could leave a stray file on failure). It builds the
  command line once, quoted correctly (only backslashes before a quote are
  doubled, so ordinary paths are not mangled), and CreateProcess launches
  the new instance directly.
- The delay moved into the new instance: a -restartwait flag makes it wait
  ~2.5s before it opens the network port and the block index, so the old
  process is gone first. No shell delay hack. Verified: -restartwait adds
  the wait (0.16s -> 2.67s).
- Linux re-execs /proc/self/exe (absolute) and only falls back to argv[0],
  which may be relative or a bare PATH name that execv cannot resolve.
- This instance now exits only if the relaunch actually started; on failure
  it keeps running and tells the user to restart manually, so a failed
  relaunch never leaves them with no node.

GUI/process wiring only; wallet code unchanged, suites still green.

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