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/measurements/index.html.erb

@@ -1,5 +1,5 @@
 <%# TODO: show hint when no quantities/units defined %>
-<div class="rightside-area buttongrid">
+<div class="rightside-area buttongrid" data-controller="measurements-view">
   <% if current_user.at_least(:active) %>
     <%= image_link_to t('.new_measurement'), 'plus-outline', new_measurement_path,
       id: :new_measurement_link, onclick: 'this.blur();',
@@ -7,16 +7,17 @@
   <% end %>
   <%= image_button_tag '', 'view-rows', name: nil, type: 'button',
     class: 'view-toggle', title: t('.view_compact'),
-    data: {view: 'compact'}, onclick: "setMeasurementsView('compact')" %>
+    data: {view: 'compact', action: 'click->measurements-view#set',
+           'measurements-view-name-param': 'compact'} %>
   <%= image_button_tag '', 'view-columns', name: nil, type: 'button',
     class: 'view-toggle', title: t('.view_wide'),
-    data: {view: 'wide'}, onclick: "setMeasurementsView('wide')" %>
-
+    data: {view: 'wide', action: 'click->measurements-view#set',
+           'measurements-view-name-param': 'wide'} %>
 </div>
 
 <div class="main-area measurements-section">
   <%= tag.div id: :measurement_edit_form %>
-  <table class="items-table measurements-compact">
+  <table class="items-table measurements-compact" data-controller="measurements">
     <thead>
       <tr>
         <th><%= Quantity.model_name.human %></th>
@@ -29,7 +30,7 @@
         <% end %>
       </tr>
     </thead>
-    <tbody id="measurements">
+    <tbody id="measurements" data-measurements-target="tbody">
       <%= render(partial: 'readout', collection: @measurements, as: :readout) || render_no_items %>
     </tbody>
   </table>
@@ -38,4 +39,3 @@
     <%= render 'wide_table', wide_groups: @wide_groups, wide_quantities: @wide_quantities %>
   </div>
 </div>
-