fix: resolve form submission issues and add CPT detail view

- Rename day/month/year form fields to umzug_day/umzug_month/umzug_year
  to avoid conflict with WordPress reserved public query variables that
  caused a 404 on form POST
- Move form handler instantiation to init_hooks() so handle_submission
  registers on the init action before it fires
- Add standalone template fallback: detect [umzugsliste] shortcode in
  page content when form_page_id option is not set
- Add submission details meta box to CPT entries showing addresses,
  furniture, additional work, and status
- Add German translations for all new admin meta box strings

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

languages/siegel-umzugsliste-de_DE.po

@@ -1065,3 +1065,75 @@ msgstr "Zimmersumme"
 #: templates/form-page.php:41
 msgid "Qty"
 msgstr "Anz."
+
+#: includes/class-cpt.php
+msgid "Submission Details"
+msgstr "Eingangsdetails"
+
+#: includes/class-cpt.php
+msgid "No submission data found."
+msgstr "Keine Eingangsdaten gefunden."
+
+#: includes/class-cpt.php
+msgid "Email sent:"
+msgstr "E-Mail gesendet:"
+
+#: includes/class-cpt.php
+msgid "Total CBM:"
+msgstr "Gesamt-CBM:"
+
+#: includes/class-cpt.php
+msgid "Addresses"
+msgstr "Adressen"
+
+#: includes/class-cpt.php
+msgid "Loading"
+msgstr "Beladeadresse"
+
+#: includes/class-cpt.php
+msgid "Unloading"
+msgstr "Entladeadresse"
+
+#: includes/class-cpt.php
+msgid "Furniture"
+msgstr "Möbel"
+
+#: includes/class-cpt.php
+msgid "Room"
+msgstr "Raum"
+
+#: includes/class-cpt.php
+msgid "Item"
+msgstr "Gegenstand"
+
+#: includes/class-cpt.php
+msgid "Elevator (Loading)"
+msgstr "Lift (Beladeadresse)"
+
+#: includes/class-cpt.php
+msgid "Elevator (Unloading)"
+msgstr "Lift (Entladeadresse)"
+
+#: includes/class-cpt.php
+msgid "Floor (Loading)"
+msgstr "Geschoss (Beladeadresse)"
+
+#: includes/class-cpt.php
+msgid "Floor (Unloading)"
+msgstr "Geschoss (Entladeadresse)"
+
+#: includes/class-cpt.php
+msgid "Fax (Loading)"
+msgstr "Telefax (Beladeadresse)"
+
+#: includes/class-cpt.php
+msgid "Fax (Unloading)"
+msgstr "Telefax (Entladeadresse)"
+
+#: includes/class-cpt.php
+msgid "Mobile (Loading)"
+msgstr "Mobil (Beladeadresse)"
+
+#: includes/class-cpt.php
+msgid "Mobile (Unloading)"
+msgstr "Mobil (Entladeadresse)"