🐛 Render no type attribute on DS buttons used as links (#12709)

* 🐛 Render no type attribute on DS buttons used as links

With `to`, button* renders an <a href> but still passed type="button". On
an anchor, type is a MIME type hint, so "button" is invalid HTML. The
type now applies only when the element is a <button> (still "button" by
default).

New "As Link" story with two tests: the link has its href and no type,
and a plain button keeps type="button".

AI-assisted-by: claude-opus-5-5

* 📚 Document the DS button as a link

The Semantics section only said that button* accepts `to`. It now says
when to use it (the control goes to another URL), what it renders (an
<a href> with the button style and no type), how to keep the in-app
navigation on a plain click, and shows the "As Link" story.

AI-assisted-by: claude-opus-5-5

frontend/src/app/main/ui/ds/buttons/button.cljs

@@ -36,7 +36,8 @@
                                      :button-destructive (= variant "destructive"))
         props (mf/spread-props props {:class [class internal-class]
                                       :href to
-                                      :type (d/nilv type "button")
+                                      ;; On an anchor, `type` is a MIME type hint.
+                                      :type (when-not to (d/nilv type "button"))
                                       :ref (fn [node]
                                              (when on-ref
                                                (on-ref node)))})]

frontend/src/app/main/ui/ds/buttons/button.stories.jsx

@@ -6,6 +6,7 @@
 
 import * as React from "react";
 import Components from "@target/components";
+import { within, expect } from "storybook/test";
 
 const { Button } = Components;
 const { icons } = Components.meta;
@@ -67,3 +68,34 @@ export const Destructive = {
     variant: "destructive",
   },
 };
+
+// With `to`, the button renders an anchor: it keeps the button look but it is
+// a real link (href, open in a new tab, announced as a link).
+export const AsLink = {
+  args: {
+    variant: "secondary",
+    to: "https://penpot.app",
+    children: "Go to Penpot",
+  },
+};
+
+export const TestAsLinkRendersAnchor = {
+  args: AsLink.args,
+  play: async ({ canvasElement }) => {
+    const link = within(canvasElement).getByRole("link", {
+      name: "Go to Penpot",
+    });
+    await expect(link).toHaveAttribute("href", "https://penpot.app");
+    // `type` on an anchor is a MIME type hint, not a button type.
+    await expect(link).not.toHaveAttribute("type");
+  },
+};
+
+export const TestButtonDefaultsToTypeButton = {
+  play: async ({ canvasElement }) => {
+    const button = within(canvasElement).getByRole("button", {
+      name: "Lorem ipsum",
+    });
+    await expect(button).toHaveAttribute("type", "button");
+  },
+};

frontend/src/app/main/ui/ds/buttons/buttons.mdx

@@ -71,8 +71,23 @@ Action buttons are a variant of `icon-button*` (`"action"`) but smaller.
 
 ### Semantics
 
-In some cases, a button needs to be semantically an 'a' tag, but visually maintain the button style.
-In that case, the button accepts a 'to' property with the expected 'href'. It will then be rendered as an anchor element.
+When the control takes the user to another page or URL, it must be a link, even if it looks like a button.
+Pass the destination in `to`: `button*` then renders an `<a href>` with the button style, so it can be
+opened in a new tab and assistive technologies announce it as a link. It renders no `type` attribute,
+which on an anchor would mean a MIME type.
+
+```clj
+[:> button* {:variant "secondary"
+             :to (rt/resolve-uri router :auth-login {})}
+ (tr "profile.recovery.go-to-login")]
+```
+
+To keep the in-app navigation on a plain click, add an `on-click` that calls `prevent-default` (unless
+Ctrl/Cmd is pressed) and emits the navigation event. Other anchor attributes, such as `target`, pass through.
+
+<Canvas of={ButtonStories.AsLink} />
+
+Use a plain `button*` (without `to`) for actions that do not change the URL, such as opening a modal.
 
 ### Accessibility