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/_form.html.erb

@@ -1,6 +1,7 @@
 <%= tabular_fields_for @unit, form: form_tag do |form| %>
-  <%- tag.tr id: row, class: "form", onkeydown: "formProcessKey(event)",
-    data: {link: link, form: form_tag, hidden_row: hidden_row} do %>
+  <%- tag.tr id: row, class: "form",
+    data: {controller: 'form', action: 'keydown->form#processKey',
+           link: link, form: form_tag, hidden_row: hidden_row} do %>
 
     <td style="--depth:<%= @unit.base_id? ? 1 : 0 %>">
       <%= form.text_field :symbol, required: true, autofocus: true, size: 12 %>