chore: Upgrade to ActiveAdmin 4.0.0.beta20 and simplify test matrix

- Update ActiveAdmin from beta16 to beta20
- Drop support for Rails 6.1, 7.0, 7.1 and ActiveAdmin 2.x/3.x
- Minimum requirements now: Rails 7.2+, Ruby 3.2+, ActiveAdmin 4.x
- Reduce CI jobs from 22+ to 6 (2 gemfiles × 3 Ruby versions)
- Clean up obsolete documentation files

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/activeadmin-4-asset-setup.md

@@ -0,0 +1,171 @@
+# ActiveAdmin 4 Asset Setup (Tailwind CSS v4)
+
+## Overview
+This guide documents the current, working setup for ActiveAdmin 4.0.0.beta19 with Tailwind CSS v4 and Tom Select (no jQuery).
+
+## Requirements
+- ActiveAdmin 4.0.0.beta19
+- Tailwind CSS v4 via `tailwindcss-rails` (ships `bin/tailwindcss`)
+- ESBuild (or other JS bundler) for ActiveAdmin JS + Tom Select
+
+## 1. Gemfile
+
+```ruby
+# ActiveAdmin 4 beta
+gem "activeadmin", "4.0.0.beta19"
+
+gem "activeadmin-tom_select"
+
+# Tailwind CSS v4 (bundled CLI)
+gem "tailwindcss-rails", "~> 4.4.0"
+```
+
+## 2. package.json
+
+```json
+{
+  "dependencies": {
+    "@activeadmin/activeadmin": "^4.0.0-beta19",
+    "tom-select": "^2.4.3",
+    "activeadmin-tom_select": "^4.1.0"
+  },
+  "devDependencies": {
+    "esbuild": "^0.27.2"
+  },
+  "scripts": {
+    "build:js": "node esbuild-active_admin.config.js",
+    "build:css": "bundle exec rake active_admin:build",
+    "build": "npm run build:js && npm run build:css",
+    "watch:js": "node esbuild-active_admin.config.js --watch",
+    "watch:css": "bundle exec rake active_admin:watch",
+    "dev": "npm run watch:js & npm run watch:css"
+  }
+}
+```
+
+## 3. JavaScript Entry (app/javascript/active_admin.js)
+
+```javascript
+import "@activeadmin/activeadmin";
+
+import TomSelect from "tom-select";
+window.TomSelect = TomSelect;
+
+import { setupAutoInit, initSearchableSelects } from "activeadmin-tom_select";
+window.initSearchableSelects = initSearchableSelects;
+setupAutoInit();
+```
+
+## 4. Tailwind Input (app/assets/stylesheets/active_admin.tailwind.css)
+
+```css
+@import "tailwindcss";
+@config "../../../tailwind-active_admin.config.mjs";
+
+@import "activeadmin-tom_select/css";
+```
+
+## 5. Tailwind Config (tailwind-active_admin.config.mjs)
+
+```javascript
+import { execSync } from "child_process";
+import activeAdminPlugin from "@activeadmin/activeadmin/plugin";
+
+const activeAdminPath = execSync("bundle show activeadmin", {
+  encoding: "utf-8"
+}).trim();
+
+export default {
+  content: [
+    `${activeAdminPath}/vendor/javascript/flowbite.js`,
+    `${activeAdminPath}/plugin.js`,
+    `${activeAdminPath}/app/views/**/*.{arb,erb,html,rb}`,
+    "./app/admin/**/*.{arb,erb,html,rb}",
+    "./app/views/active_admin/**/*.{arb,erb,html,rb}",
+    "./app/views/admin/**/*.{arb,erb,html,rb}",
+    "./app/views/layouts/active_admin*.{erb,html}",
+    "./app/javascript/**/*.js"
+  ],
+  darkMode: "selector",
+  plugins: [activeAdminPlugin]
+};
+```
+
+## 6. Tailwind Build Tasks (lib/tasks/active_admin.rake)
+
+```ruby
+namespace :active_admin do
+  desc "Build Active Admin Tailwind stylesheets"
+  task build: :environment do
+    command = [
+      Rails.root.join("bin/tailwindcss").to_s,
+      "-i", Rails.root.join("app/assets/stylesheets/active_admin.tailwind.css").to_s,
+      "-o", Rails.root.join("app/assets/builds/active_admin.css").to_s,
+      "-m"
+    ]
+
+    system(*command, exception: true)
+  end
+
+  desc "Watch Active Admin Tailwind stylesheets"
+  task watch: :environment do
+    command = [
+      Rails.root.join("bin/tailwindcss").to_s,
+      "--watch",
+      "-i", Rails.root.join("app/assets/stylesheets/active_admin.tailwind.css").to_s,
+      "-o", Rails.root.join("app/assets/builds/active_admin.css").to_s,
+      "-m"
+    ]
+
+    system(*command, exception: true)
+  end
+end
+
+Rake::Task["assets:precompile"].enhance(["active_admin:build"])
+Rake::Task["test:prepare"].enhance(["active_admin:build"]) if Rake::Task.task_defined?("test:prepare")
+Rake::Task["spec:prepare"].enhance(["active_admin:build"]) if Rake::Task.task_defined?("spec:prepare")
+Rake::Task["db:test:prepare"].enhance(["active_admin:build"]) if Rake::Task.task_defined?("db:test:prepare")
+```
+
+## 7. Tailwind CLI Binstub
+
+```bash
+bundle binstubs tailwindcss-ruby --force
+```
+
+## 8. Upgrade Notes (ActiveAdmin 4.0.0.beta19)
+
+From ActiveAdmin's upgrade guide:
+- `_site_header.html.erb` container class changed from `sticky` to `fixed`.
+- `active_admin.html.erb` adds the `pt-16` utility class.
+- Tailwind v4 uses `@import "tailwindcss"` and `@config` (no `@tailwind` directives).
+- The Tailwind config file is ESM. Use `tailwind-active_admin.config.mjs`.
+- jQuery/jQuery UI removed; `columns` and `tabs` components removed.
+- Replace `default_main_content` with `render "show_default"`.
+- Replace `as: :datepicker` with `as: :date_picker`.
+- Replace `active_admin_comments` with `active_admin_comments_for(resource)`.
+- Replace `attributes_table` with `attributes_table_for(resource)` in sidebars.
+
+## Troubleshooting
+
+### CSS not building
+- Ensure `tailwindcss-rails` is pinned to `~> 4.4.0`.
+- Run `bundle binstubs tailwindcss-ruby --force`.
+- Confirm `bin/tailwindcss` exists.
+
+### ActiveAdmin styles not loading
+- Verify `app/assets/builds/active_admin.css` exists.
+- Check that `active_admin.tailwind.css` imports `activeadmin-tom_select/css`.
+
+### Tom Select not initializing
+- Verify `window.TomSelect` in the console.
+- Ensure `setupAutoInit()` is called.
+
+## Migration Checklist
+
+- [ ] Update ActiveAdmin to 4.0.0.beta19
+- [ ] Use Tailwind v4 `@import` + `@config`
+- [ ] Add `tailwindcss-rails` (~> 4.4.0) and binstub
+- [ ] Create `tailwind-active_admin.config.mjs` (ESM)
+- [ ] Update rake tasks to use `bin/tailwindcss` (no `-c`)
+- [ ] Rebuild assets and verify UI