fix(post-form): convert HEIC via FilePond beforeAddFile, not a custom uploader (U4)

Runtime verification showed the custom picker uploaded to Grav's flash but
never attached photos to the entry — attachment needs FilePond's exact
(undocumented) submit contract. Reverting to type:filepond and hooking its
beforeAddFile: a HEIC item is rejected, converted to JPEG via the lazy heic-to
chunk, then re-added with pond.addFile() so FilePond owns upload + page-attach
(the proven path). Web-format photos pass through; conversion failures fail
closed (inline status, original never added). Removes the custom photos field
template + AJAX uploader.

Verified end-to-end in a browser: HEIC posts as JPEG, corrupt HEIC is skipped,
Submit gated while converting, draft photos-reselect hint intact.

Refs R8, R9, R16, R17, AE1, AE4, KTD4.

themes/intotheeast/templates/forms/fields/photos/photos.html.twig

@@ -1,28 +0,0 @@
-{#
-  Custom `photos` form field — a plain, controlled picker that replaces Grav's
-  managed filepond/dropzone field on /post so post-form.js can convert HEIC
-  before upload (see U4). It carries the same destination/accept/limit settings
-  the server's uploadFiles() reads via the blueprint, but renders its own input
-  and uploads through post-form.js, not FilePond.
-#}
-{% extends "forms/field.html.twig" %}
-
-{% block input %}
-    {% set files = config.plugins.form.files|merge(field|default([])) %}
-    {% set limit = not field.multiple ? 1 : (field.limit ?? files.limit ?? 4) %}
-    <div class="photo-picker {{ field.classes }}"
-         data-file-field-name="{{ field.name }}"
-         data-file-url-add="{{ form.getFileUploadAjaxRoute().getUri()|e('html_attr') }}"
-         data-limit="{{ limit }}">
-        <label class="photo-picker__add btn-action">
-            <span class="photo-picker__add-label">📷 Add photos</span>
-            <input type="file"
-                   class="photo-picker__input"
-                   accept="image/*,.heic,.heif,.HEIC,.HEIF"
-                   {% if field.multiple %}multiple="multiple"{% endif %}
-                   hidden />
-        </label>
-        <ul class="photo-picker__list" aria-live="polite"></ul>
-        <p class="photo-picker__hint form-status" role="status"></p>
-    </div>
-{% endblock %}