contact messages: settings subject input presented as 'topic'

Password managers mistake an input named 'subject' (sitting next to the
email box) for a credential field and nag with autofill regardless of
autocomplete settings; the placeholder/tooltips now say 'topic'. The
contact_subject data/API field is unchanged.

frontend/src/SiteEditor.vue

@@ -768,7 +768,7 @@ onUnmounted(() => {
         <LangSelect
           v-model="lang"
           :options="contactLangOptions"
-          title="Language of the label/subject strings (auto translators fill these too; this edits the manual override)"
+          title="Language of the label/topic strings (auto translators fill these too; this edits the manual override)"
         />
         <input
           v-model="contactLabel"
@@ -788,8 +788,8 @@ onUnmounted(() => {
         <input
           v-model="contactSubject"
           class="text-input"
-          :placeholder="translating ? contactSubjectOrig : 'subject'"
-          title="Subject for the mailto link (empty = the label)"
+          :placeholder="translating ? contactSubjectOrig : 'topic'"
+          title="Topic for the mailto link (empty = the label)"
           @input="onContactInput"
         />
       </div>
@@ -926,7 +926,10 @@ onUnmounted(() => {
 }
 
 /* The messaging block: one line — the article editor's flag selector,
-   then small text boxes (label = the enable switch, email, subject). */
+   then small text boxes (label = the enable switch, email, topic). The
+   mailto subject is labeled "topic" in the UI: password managers
+   mistake an input named "subject" for a credential field and offer
+   autofill no matter the autocomplete setting. */
 .msg-head {
   flex-wrap: wrap;
 }