fix(a11y): inline placeholder title uses <p>, not <h4>

[danspringer]

Summary

The inline placeholder rendered by InlineConsent::renderPlaceholderHTML() hard-wires a <h4> for the service title (fragments/ConsentManager/inline_placeholder.php). When the placeholder is rendered inside arbitrary page content, this almost always breaks the document outline: a <section> already running on <h2> / <h3> suddenly contains a stray <h4> with no parent heading. Lighthouse and other a11y audits flag this as a heading-order issue (we noticed it dropping our score on a fully-clean page from 100 to 98).

This PR switches the tag to <p> so the placeholder no longer participates in the heading outline, while keeping .consent-inline-title plus the Bootstrap-style .h4 utility class so themes that style by either selector stay visually identical.

Change

- <h4 class="consent-inline-title">…</h4>
+ <p class="consent-inline-title h4">…</p>

• assets/consent_inline.css targets .consent-inline-title (class only, not tag) → no CSS update needed, visual rendering unchanged.
• The bundled box_bootstrap5.php already uses the same class="… h4 …" pattern on non-heading elements, so adding .h4 here is consistent with the addon.
• Themes that do want a real heading can still override the fragment via theme/private/fragments/ConsentManager/inline_placeholder.php (REDAXO fragment override).

Test plan

• Inline placeholder still renders the title text in the same visual style across the bundled box variants (default, bootstrap5, uikit3, bulma).
• Lighthouse "Heading elements appear in a sequentially-descending order" no longer flags pages embedding the placeholder.
• Existing CSS overrides targeting .consent-inline-title keep working (class-only selectors).