feat(120): alert component unification

Phase 120 - Plan 01:
- Reusable PHP alert component (resources/views/components/alert.php)
  with inline SVG icon per type, optional dismiss button.
- Missing .alert--info SCSS variant added (#eff6ff/#bfdbfe/#1e3a8a) -
  fixes black-on-white alert after Fakturownia test connection.
- Flash::push(type, message) + Flash::all() with BC for set/get;
  legacy key heuristic (error/.save/warning -> typed entries).
- Central flash renderer in 3 layouts (app/auth/public) iterating
  Flash::all() through component (.alerts-stack wrap).
- Vanilla JS alert-dismiss.js with idempotent guard and delegated
  click handler on [data-alert-dismiss].
- 36 views migrated off inline <div class="alert alert--TYPE">;
  .flash--error/success removed from views (orders/show, shipments/prepare).
- SCSS rebuilt: public/assets/css/{app,login}.css.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

resources/views/accounting/invoice_form.php

@@ -30,11 +30,13 @@ $buyerEmailDefault = trim((string) ($buyerAddr['email'] ?? $orderRow['buyer_emai
   </div>
 
   <?php if ($errorMsg !== ''): ?>
-    <div class="alert alert--danger mt-12"><?= $e($errorMsg) ?></div>
+    <div class="mt-12"><?php $type='danger'; $message=(string) $errorMsg; $dismissible=true; include dirname(__DIR__) . '/components/alert.php'; ?></div>
   <?php endif; ?>
 
   <?php if ($hasExistingInvoices): ?>
-    <div class="alert alert--warning mt-12">
+    <?php
+      ob_start();
+      ?>
       <strong>Uwaga!</strong> Do tego zamowienia wystawiono juz <?= $e((string) count($existingInvoicesList)) ?> fakture/y:
       <ul class="mt-4">
         <?php foreach ($existingInvoicesList as $ei): ?>
@@ -46,7 +48,10 @@ $buyerEmailDefault = trim((string) ($buyerAddr['email'] ?? $orderRow['buyer_emai
           </li>
         <?php endforeach; ?>
       </ul>
-    </div>
+      <?php
+      $existingInvoicesHtml = ob_get_clean();
+    ?>
+    <div class="mt-12"><?php $type='warning'; $messageHtml = $existingInvoicesHtml; $dismissible=true; include dirname(__DIR__) . '/components/alert.php'; unset($messageHtml); ?></div>
   <?php endif; ?>
 
   <form id="invoice-create-form" method="post" action="/orders/<?= $e((string) $orderIdVal) ?>/invoice/store" class="mt-16">