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>

Gemfile.lock

@@ -273,6 +273,8 @@ GEM
     sqlite3 (2.9.0-x86_64-darwin)
     sqlite3 (2.9.0-x86_64-linux-gnu)
     sqlite3 (2.9.0-x86_64-linux-musl)
+    stimulus-rails (1.3.4)
+      railties (>= 6.0.0)
     stringio (3.2.0)
     thor (1.5.0)
     tilt (2.7.0)
@@ -324,6 +326,7 @@ DEPENDENCIES
   selenium-webdriver
   sprockets-rails
   sqlite3 (~> 2.7)
+  stimulus-rails
   turbo-rails (~> 2.0)
   tzinfo-data
   web-console