Add dark mode with toggle and localStorage persistence

Uses Tailwind v4 class-based dark mode with a Stimulus controller for
toggling. Respects prefers-color-scheme as default, prevents FOUC with
an inline script, and persists the user's choice in localStorage. All
views updated with dark: variants for backgrounds, text, borders,
badges, buttons, and form inputs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/javascript/controllers/dark_mode_controller.js

@@ -0,0 +1,27 @@
+import { Controller } from "@hotwired/stimulus"
+
+export default class extends Controller {
+  static targets = ["icon"]
+
+  connect() {
+    this.updateIcon()
+  }
+
+  toggle() {
+    document.documentElement.classList.toggle("dark")
+    const isDark = document.documentElement.classList.contains("dark")
+    localStorage.setItem("theme", isDark ? "dark" : "light")
+    this.updateIcon()
+  }
+
+  updateIcon() {
+    const isDark = document.documentElement.classList.contains("dark")
+    this.iconTargets.forEach(icon => {
+      if (icon.dataset.mode === "dark") {
+        icon.classList.toggle("hidden", !isDark)
+      } else {
+        icon.classList.toggle("hidden", isDark)
+      }
+    })
+  }
+}