feat(post-form): client-side HEIC->JPEG photo picker (U4)

Replace /post's managed filepond field with a controlled 'photos' field
(theme forms/fields/photos) + picker logic in post-form.js: magic-byte
sniff (ISO-BMFF ftyp brands, not filename/MIME), lazy import('heic-to')
only for real HEIC (deferred 3MB chunk via ESM splitting), per-thumbnail
converting/uploading/done/error states, in-flight counter gating Submit,
and fail-closed skip on conversion failure. Converted JPEGs POST to Grav's
AJAX file-upload route into the form flash (the only path copyFiles reads),
so add-page-by-form attaches them on submit. Web-format photos pass through.

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

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

@@ -0,0 +1,28 @@
+{#
+  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 %}