feat(errors): erori pe 3 niveluri (problema+cauza+fix) pe API si UI (PRD 5.4)

Catalog central pur app/errors.py ca sursa unica cod->{problema,fix},
consumat de API+UI+worker. Aditiv (field/message pastrate la octet) +
rar_error stocat superset. Scope: fluxul de declarare; login/signup/CSRF
neatinse. labels.parse_erori degradeaza gratios; UI progresiv AA light+dark.
631 teste.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/web/templates/_mapcoloane.html

@@ -1,12 +1,17 @@
 <div id="import-section">
   {% set pas = 2 %}{% include '_stepper.html' %}
+  {% from '_eroare.html' import card_erori %}
   <div class="card">
     <h2 style="font-size:15px; margin:0 0 12px;">
       Mapare coloane —
       <span class="muted" style="font-weight:400;">{{ filename or ("import #" ~ import_id) }}</span>
     </h2>
 
-    {% if message %}
+    {% if eroare_mapare %}
+    <div style="margin-bottom:12px;">
+      {{ card_erori([eroare_mapare]) }}
+    </div>
+    {% elif message %}
     <div class="flash" style="{% if error %}border-color:var(--err); background:color-mix(in srgb, var(--err) 12%, var(--card));{% endif %} margin-bottom:12px;"
          {% if error %}role="alert"{% endif %}>
       {{ message }}