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/readouts/_form.html.erb

@@ -1,6 +1,6 @@
-<%# TODO: add readout reordering by dragging %>
 <%= tabular_fields_for 'readouts[]', readout do |form| %>
-  <%- tag.tr id: dom_id(readout.quantity, :new, :readout) do %>
+  <%- tag.tr id: dom_id(readout.quantity, :new, :readout),
+             data: {controller: 'readout-unit'} do %>
     <td>
       <%# TODO: add grayed readout index (in separate column?) %>
       <%= readout.quantity.relative_pathname(@superquantity) %>
@@ -13,16 +13,18 @@
       <%= form.collection_select :unit_id, @user_units, :id,
         ->(u){ sanitize('&emsp;' * (u.base_id ? 1 : 0) + u.symbol) },
         {prompt: '', disabled: '', selected: readout.quantity.default_unit_id || ''}, required: true,
-        data: {default_unit_id: readout.quantity.default_unit_id || ''},
-        onchange: "readoutUnitChanged(this)" %>
+        data: {default_unit_id: readout.quantity.default_unit_id || '',
+               readout_unit_target: 'select',
+               action: 'change->readout-unit#unitChanged'} %>
     </td>
     <td class="flex">
       <%# TODO: change to _link_ after giving up displaying relative paths %>
       <%= image_button_tag '', 'check-circle-outline',
         class: 'set-default-unit', name: nil, type: 'button', disabled: true,
         title: t('readouts.form.set_default_unit'),
-        data: {path: quantity_path(readout.quantity)},
-        onclick: 'setDefaultUnit(this)' %>
+        data: {path: quantity_path(readout.quantity),
+               readout_unit_target: 'button',
+               action: 'click->readout-unit#setDefault'} %>
       <%= image_button_tag '', 'delete-outline', class: 'dangerous', name: nil,
         formaction: discard_readouts_path(readout.quantity),
         formmethod: :get, formnovalidate: true, data: {turbo_stream: true} %>