feat(16.2-01): translatable plugin fields, element uploads and save notices in the admin

- MLPluginField wraps PluginField in the MLField locale switcher; the
  element gets the active content locale and that locale's string
- isMLField treats a translatable plugin field as a locale map in the form
  state, FormView adopt and the relation child modal
- summer-upload posts the file with the admin session to the field upload
  route and answers summer-upload-result {id, url} or {id, error}
- noticeLines shows meta.notices under each field as a warning status line
  (text only) and in the saved toast; the stored value is reloaded
- docs/backend/field-types.md documents the full element contract;
  modules/boardwalk/dist rebuilt

admin/tests/form/formState.test.ts

@@ -8,6 +8,7 @@ import {
   focusField,
   initialValues,
   mergeMLValue,
+  noticeLines,
   panelDomId,
   presetUpdates,
   presetValue,
@@ -302,3 +303,24 @@ describe('recordDisplayName', () => {
     expect(recordDisplayName(fields, { notify: true, tags: [1], title: { en: 'Post', pl: '' }, name: 'Anna' }, locales)).toBe('Post')
   })
 })
+
+describe('noticeLines', () => {
+  it('maps notices to lines per field, prefixing the content locale', () => {
+    expect(
+      noticeLines([
+        { field: 'content', locale: 'pl', message: '2 elements were removed: style, iframe' },
+        { field: 'content', locale: 'en', message: 'A script was removed' },
+        { field: 'notes', message: 'Trimmed' },
+      ]),
+    ).toEqual({
+      content: ['PL: 2 elements were removed: style, iframe', 'EN: A script was removed'],
+      notes: ['Trimmed'],
+    })
+  })
+
+  it('is empty without notices', () => {
+    expect(noticeLines(undefined)).toEqual({})
+    expect(noticeLines([])).toEqual({})
+    expect(noticeLines([{ field: 'x', message: '' }])).toEqual({})
+  })
+})