feat(post-form): native datetime-local picker + require date client-side

The stock Grav `datetime` field template is deprecated and falls back to a
plain text box, so `type: datetime` + `default: now` rendered a raw input
showing the literal word "now" — unusable. Add a theme override at
templates/forms/fields/datetime/datetime.html.twig that renders a native
<input type="datetime-local"> (real calendar+clock, great on mobile), drop
the `default: now`, and prefill the current local time from post-form.js.

Also add `date` to the existing client-side validator. Together with the
picker (which can't hold an invalid value) this stops a bad/empty date from
round-tripping to the server — which was the trigger that made Grav re-render
the managed FilePond field from the session flash as filename-only inputs and
resurrect a photo the user had removed. Grav still reformats the submitted
value to the blueprint `format: 'Y-m-d H:i'` on save, so stored dates and
folder slugs are unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -0,0 +1,18 @@
+{# intotheeast override of Grav's datetime field.
+
+   Grav's stock forms/fields/datetime/datetime.html.twig is DEPRECATED and just
+   extends the text field, so `type: datetime` renders a plain <input type="text">
+   — the owner sees a raw box (the literal word "now" from `default: now`) with no
+   way to pick a date/time. Here we render a native <input type="datetime-local">
+   instead: a real calendar + clock picker, and an excellent one on mobile.
+
+   The value is prefilled to the current local time by post-form.js (JS knows the
+   traveller's timezone; the server doesn't), and the existing client-side
+   validator now requires this field — so an empty or invalid date can no longer
+   reach the server and destroy the FilePond photo list on a re-render. #}
+{% extends "forms/field.html.twig" %}
+
+{% block input_attributes %}
+    type="datetime-local"
+    {{ parent() }}
+{% endblock %}