feat(ssr): subtle fade-in on wizard step cards

§3 animations: the migrated wizard step cards had no transition, so step→step
swaps and modal open were flat. The old mm/* slide system was deleted in Phase
11 (and its classes purged from CSS), and the transaction-edit "reference" uses
an undefined `last-modal-step` no-op — so there was no clean slide to restore.

Apply the codebase's existing `fade-in transition-opacity duration-300`
primitive (`.htmx-added .fade-in` in input.css) to all three wizard step cards
(new-invoice basic-details + accounts, vendor step-card, client step-card). Each
card now fades in on open and on every step swap. Verified live: cards always
settle to opacity 1 (never stuck invisible) on both open and step navigation.

Richer directional (forward/back) slide transitions are left for a design pass.

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

src/clj/auto_ap/ssr/invoice/new_invoice_wizard.clj

@@ -333,7 +333,7 @@
           client-val (or (:invoice/client data) client-from-req)]
       (com/modal-card-advanced
        {"@keydown.enter.prevent.stop" "if ($refs.next) {$refs.next.click()}"
-        :class " md:w-[750px] md:h-[600px] w-full h-full"
+        :class " md:w-[750px] md:h-[600px] w-full h-full fade-in transition-opacity duration-300"
         "x-data" ""}
        (com/modal-header {} [:div.p-2 (if extant? "Edit invoice" "New invoice")])
        (com/modal-body
@@ -526,7 +526,7 @@
           total (expense-accounts-total* rows)]
       (com/modal-card-advanced
        {"@keydown.enter.prevent.stop" "if ($refs.next) {$refs.next.click()}"
-        :class " md:w-[750px] md:h-[600px] w-full h-full"
+        :class " md:w-[750px] md:h-[600px] w-full h-full fade-in transition-opacity duration-300"
         "x-data" ""}
        (com/modal-header {} [:div.p-2 "Invoice accounts "])
        (com/modal-body