feat(post-form): require 1–6 photos per entry

Raise the FilePond limit from 4 to 6 and enforce a minimum of one photo.
The photo field is first in the form, so initValidation checks it first: an
empty picker blocks submit, reveals the (possibly collapsed) photo section,
and shows "Add at least one photo." under its header. Labels updated to
"Photos (1–6)".

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

pages/02.post/post-form.md

@@ -24,7 +24,7 @@ form:
         # bar so the thumbnails don't crowd the writing area.
         -
             name: photos
-            label: Photos (max 4)
+            label: Photos (1–6)
             # Grav-managed filepond field. post-form.js hooks its beforeAddFile
             # to convert HEIC->JPEG in the browser, then re-adds the JPEG via the
             # public pond.addFile() API — FilePond keeps ownership of the upload
@@ -32,7 +32,9 @@ form:
             type: filepond
             multiple: true
             destination: '@self'
-            limit: 4
+            # Max 6 photos per entry; at least 1 is required (enforced
+            # client-side in post-form.js initValidation).
+            limit: 6
             accept:
                 - 'image/*'