Style the coach news form's photo picker, reword the footer note

The bare native file input (browser-chrome "Choose Files" button) clashed
with every other styled field on the screen -- new .m-file-input component
class mirrors management's own .file-input treatment, recolored for the
mobile palette. Also "Linked to {team}" reads clearer than "Posted for
{team}" now that the field is a hard-locked link, not a free-text audience.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

mobile/coach_views.py

@@ -668,6 +668,7 @@ class CoachCreateNewsView(CoachScopeMixin, LoginRequiredMixin, TemplateView):
     def build_photo_form(self, data=None, files=None):
         photo_form = NewsPhotoUploadForm(data, files)
         photo_form.fields["images"].required = False
+        photo_form.fields["images"].widget.attrs["class"] = "m-file-input"
         return photo_form
 
     def get_context_data(self, **kwargs):