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/users/sessions/new.html.erb

@@ -1,5 +1,5 @@
 <%= labeled_form_for resource, url: user_session_path,
-  html: {class: 'main-area', onsubmit: 'formValidate(event)'} do |f| %>
+  html: {class: 'main-area', data: {controller: 'form', action: 'submit->form#validate'}} do |f| %>
 
   <%= f.email_field :email, required: true, size: 30, autofocus: true,
     autocomplete: 'email' %>