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>

pages/02.post/post-form.md

@@ -46,8 +46,12 @@ form:
         -
             name: date
             label: Date & Time
+            # Rendered as a native <input type="datetime-local"> by the theme
+            # override at templates/forms/fields/datetime/datetime.html.twig.
+            # No `default: now` — the stock text fallback would print the literal
+            # string "now" into the picker; post-form.js prefills the current
+            # local time instead (and the client validator requires this field).
             type: datetime
-            default: now
             format: 'Y-m-d H:i'
             validate:
                 required: true