feat(ssr): restore forward/back step slide in the wizard engine

The engine migration replaced the old mm/* modal-stack wizards (which slid
forward/back between steps) with wizard2, but never carried the slide over — step
transitions went flat. Restore the original mechanism in the shared engine so all
wizards (new-invoice, vendor, client, pay, transaction-rule) get it:

- wizard2/step-slide-classes: the group-[.forward]/transition:htmx-* and
  group-[.backward]/* slide variants, applied to the swapped <form>.
- wizard2/transitioner: the #transitioner wrapper whose @htmx:after-request hook
  reads the x-transition-type response header and toggles group/transition +
  forward|backward on itself. All 5 configs' :open-response now use it.
- wizard2/handle-step-submit sets x-transition-type (forward on advance, backward
  on Back, none on a same-step validation re-render) + HX-reswap "outerHTML
  swap:0.16s" so the slide-out plays before the swap. Direction computed from
  step order (transition-type).
- Removed the interim per-card fade-in in favor of this.
- Rebuilt output.css so the 16 fwd + 16 back slide variants are compiled.

REPL-verified: open-wizard emits the transitioner, the form carries the slide
classes, and submit responses carry the transition headers. Live verification
needs a server refresh (the dev server froze its route table at startup).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/clj/auto_ap/ssr/admin/transaction_rules.clj

@@ -827,7 +827,7 @@
    ;; new/edit routes are just (partial wizard2/open-wizard config) -- no hand-rolled
    ;; create!/render/wrap/thread boilerplate.
    :open-response (fn [form]
-                    (modal-response [:div#transitioner.flex-1 form]))
+                    (modal-response (wizard2/transitioner form)))
    :steps [{:key :edit
             :decode decode-rule-form
             :validate rule-form-errors