Migrate all inline JS to Stimulus controllers

Add stimulus-rails gem and wire up 7 controllers:
- measurements_view_controller: view toggle (compact/wide) via localStorage
- measurements_controller: grouped rows MutationObserver
- charts_controller: Plotly chart rendering
- form_controller: keyboard shortcuts (Escape/Enter) and submit validation
- details_controller: quantity picker state, focusout close, MutationObserver
- readout_unit_controller: default unit button enable/disable + PATCH submission
- drag_controller: drag-and-drop for quantity reparenting and unit rebasing

Remove all inline onclick/onkeydown/ondrag*/onsubmit handlers from templates.
Remove all window.* global exports from application.js.
Remove bare <script> block from measurements/_form.html.erb.
Remove turbo:load listeners for behavior now in controller connect().

application.js now only contains: Turbo Stream custom action definitions
and the showPage visibility listener.

Document Stimulus conventions in CLAUDE.md.

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

app/views/units/index.html.erb

@@ -22,9 +22,10 @@
       <% end %>
     </tr>
     <%= tag.tr id: "unit_", hidden: true,
-               ondragover: "dragOver(event)", ondrop: "drop(event)",
-               ondragenter: "dragEnter(event)", ondragleave: "dragLeave(event)",
-               data: {drop_id: "unit_", drop_id_param: "unit[base_id]"} do %>
+               data: {controller: 'drag',
+                      action: 'dragover->drag#over drop->drag#drop dragenter->drag#enter dragleave->drag#leave',
+                      drag_drop_id_value: 'unit_',
+                      drag_drop_id_param_value: 'unit[base_id]'} do %>
       <th colspan="5"><%= t '.top_level_drop' %></th>
     <% end %>
   </thead>