feat(post-form): reverse-geocode City/Country + drop hero-image field

Get Location now reverse-geocodes the captured coordinates into City +
Country via BigDataCloud's free client endpoint, filling only blank fields
(never clobbering a manual entry) and appending the resolved place to the
location status. Best-effort — a failure leaves the coordinates intact.

Removes the redundant hero_image field: journal entries render their hero
from the first uploaded photo (entry-journal uses entry.media.images|first),
so an explicit hero filename served no purpose. Stories still use hero_image
but are not posted through this form.

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

pages/02.post/post-form.md

@@ -129,15 +129,11 @@ form:
                 'car': '🚗 Car'
                 'plane': '✈️ Plane'
 
-        # Advanced fields — collapsed behind "More options" (see U5)
-        -
-            name: hero_image
-            label: Hero Image Filename
-            type: text
-            classes: advanced-field
-            placeholder: 'photo.jpg'
-            help: 'Filename of the hero/header image. Leave blank to use the first uploaded photo.'
-
+        # Advanced fields — collapsed behind "More options" (see U5).
+        # No hero_image field: journal entries render their hero from the first
+        # uploaded photo (entry-journal.html.twig uses entry.media.images|first),
+        # so an explicit hero filename was redundant. Stories still use hero_image
+        # but they aren't posted through this form.
         -
             name: force_connect
             label: Force connector line