add progressive enhancement guidelines to CLAUDE.md

Document the CSS :has(:checked) pattern for forms with dynamic
sections, noscript fallbacks for JS-only elements, and the HEEx
<style> raw text gotcha.

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

CLAUDE.md

@@ -177,6 +177,48 @@ socket |> assign(form: to_form(changeset))
 - Avoid LiveComponents unless you have a specific need (isolated state, targeted updates)
 - Never use deprecated `phx-update="append"` or `phx-update="prepend"`
 
+## Progressive Enhancement
+
+Build HTML+CSS that works without JS. LiveView layers on top.
+
+**Principle:** If a state change is purely visual (which radio is selected, which panel is shown), handle it in CSS. Don't round-trip to the server for something the browser can do alone.
+
+**Pattern for forms with dynamic sections:**
+
+1. Render ALL possible states in the HTML (e.g. all adapter configs, all option panels)
+2. Use CSS `:has(:checked)` to show the matching section and hide the rest
+3. Namespace field names to avoid clashes (`email[brevo][api_key]`, not `email[api_key]`)
+4. Give the form both `action` (no-JS POST) and `phx-submit` (LiveView)
+5. Add a thin controller to handle the no-JS POST path
+
+```css
+/* Hide all sections by default, show the one matching the checked radio */
+.config-section[data-option] { display: none; }
+form:has(#option-foo:checked) [data-option="foo"] { display: block; }
+```
+
+**JS-only elements:** Wrap in a container with an ID, hide it via `<noscript><style>`, and provide a form POST alternative:
+
+```heex
+<span id="async-action">
+  <.button type="button" phx-click="do_thing">Do thing</.button>
+</span>
+<noscript>
+  <style>#async-action { display: none; }</style>
+  <.form for={%{}} action={~p"/thing"} method="post" style="display:inline">
+    <.button>Do thing</.button>
+  </.form>
+</noscript>
+```
+
+**HEEx and `<style>` tags:** HEEx treats `<style>` content as raw text (no `{@var}` interpolation). To output dynamic CSS, use a helper that returns `Phoenix.HTML.raw/1`:
+
+```elixir
+defp dynamic_style(value) do
+  Phoenix.HTML.raw("<style>.thing[data-x=\"#{value}\"] { display: block; }</style>")
+end
+```
+
 ## JS/CSS Guidelines
 
 - Project is fully Tailwind-free — hand-written CSS with `@layer`, native nesting, `oklch()`