Replace render_turbo_stream cancel handlers with form#cancel Stimulus action

Add cancel() method to form_controller that handles all close cases:
- tabular forms (<tr>): removes inner form, re-enables link, shows hidden row
- measurements create form: removes self, enables trigger link, shows no-items

Data attributes already present on controller elements (data-form, data-link,
data-hidden-row) drive the behaviour. Two extra attributes on the create form
(data-cancel-enable, data-cancel-show) cover the non-tabular case.

Delete now-unused _form_close and _edit_form_close partial templates.

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

app/views/units/_form.html.erb

@@ -16,7 +16,7 @@
     <td class="flex">
       <%= form.button %>
       <%= image_link_to t(:cancel), "close-outline", units_path, class: 'dangerous',
-        name: :cancel, onclick: render_turbo_stream('form_close', {row: row}) %>
+        name: :cancel, data: {action: 'click->form#cancel'} %>
     </td>
     <td></td>
   <% end %>