Add admin form field partials and apply to add/edit forms

Four reusable PHP partials extracted to templates/partials/form/:

- text-field.php  — single-line input (text/number/url); wraps input+hint in div,
                    skips the inner wrapper when no hint is present. Supports $type,
                    $placeholder, $required, $attrs, $hint, $id overrides.
- select-field.php — <select> with leading empty option; matches $selected against
                    option id OR option name string (handles view-sourced data where
                    orientation/ap/finality come back as name strings, not FK ids).
- checkbox-list.php — checkbox group (languages, formats); renders .admin-checkbox-list
                    with typed-string comparison so int ids from DB match string values.
- file-field.php  — file input with accept/multiple/hint; appends [] to name when
                    $multiple is true.

Both add.php and edit.php rewritten to use the partials:
- ~15 repeated text-field divs collapsed to single-line include calls
- ~6 repeated select divs collapsed to single-line include calls
- 4 checkbox-list blocks collapsed to 2 calls each
- 3 file input blocks collapsed to single-line include calls
- Textarea fields (synopsis, context_note) kept inline — no partial for <textarea>
- Banner preview block in edit.php kept inline — conditional UI not generalised

Line count: add.php 251→93 (-158), edit.php 289→171 (-118)

templates/partials/form/file-field.php

@@ -0,0 +1,33 @@
+<?php
+/**
+ * File input partial.
+ *
+ * Variables consumed:
+ *   string      $name      — input name attribute (used for id too unless $id set)
+ *   string      $label     — visible label text
+ *   string      $accept    — MIME types / extensions for the accept attribute (e.g. 'image/jpeg,image/png')
+ *   string|null $hint      — optional hint shown in <small> below the input
+ *   bool        $multiple  — whether to allow multiple file selection; default false
+ *   string|null $id        — override the id attribute (defaults to $name)
+ */
+
+$accept   = $accept   ?? '';
+$hint     = $hint     ?? null;
+$multiple = $multiple ?? false;
+$id       = $id       ?? $name;
+?>
+<div>
+    <label for="<?= htmlspecialchars($id) ?>"><?= htmlspecialchars($label) ?></label>
+    <div class="admin-file-input">
+        <input type="file"
+               id="<?= htmlspecialchars($id) ?>"
+               name="<?= htmlspecialchars($name) ?><?= $multiple ? '[]' : '' ?>"
+               <?= $accept   ? 'accept="'   . htmlspecialchars($accept)  . '"' : '' ?>
+               <?= $multiple ? 'multiple' : '' ?>>
+        <?php if ($hint): ?>
+        <small><?= htmlspecialchars($hint) ?></small>
+        <?php endif; ?>
+    </div>
+</div>
+<?php
+unset($accept, $hint, $multiple, $id);