Make the button-shaped links actual links

"Web oficial" did nothing when clicked, and it was not alone: six places
wrapped `<pi-action-button>` in an `<a>`. That component renders a
`<button>`, so each one was a button inside a link -- invalid HTML, and
the button swallows the click. Besides "Web oficial" it covered Demo,
Repositorio, the contact CTA on every detail page, the 404's way home
and a call card's apply button.

They become one `app-link-button`, which renders a real anchor. The
click was only part of it: a link can also be opened in a new tab,
middle-clicked, copied, tabbed to and followed by a crawler.

Its look does not come from @pi/ui's button classes, and not for want of
trying: those rules are scoped to the element (`html button:not(…)` in
`_pi-button.scss`), so on an anchor they contribute nothing except
`pi-color__primary` -- a *text* colour, which painted the label violet on
a violet fill. The stylesheet reads the same `--pi-theme-button-*`
tokens the library reads instead, so a change of shell or theme still
moves these buttons and dark mode needs nothing of its own.

Verified: the anchor reports href, target and a real navigation to the
external site; no `<button>` inside an `<a>` remains on the page; and
the rendered pill matches the header's button in radius, gradient fill,
white text and weight, in both themes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

landing/src/app/ui/call-card.ts

@@ -16,6 +16,7 @@ import type { Copy } from '../core/copy';
 import type { Locale } from '../core/api';
 import type { PublicCall } from '../core/api.types';
 import { TechnologyChip } from './technology-chip';
+import { LinkButton } from './link-button';
 
 @Component({
   selector: 'app-call-card',
@@ -32,6 +33,7 @@ import { TechnologyChip } from './technology-chip';
     PiTextColorDirective,
     PiTextWeightBoldDirective,
     TechnologyChip,
+    LinkButton,
   ],
   template: `
     <pi-card
@@ -96,15 +98,13 @@ import { TechnologyChip } from './technology-chip';
           <!-- To the call's own page and its form when it has one; to the
                landing's general form otherwise. -->
           @if (call().path; as path) {
-            <a class="landing-card__button-link" [href]="path">
-              <pi-action-button
-                size="small"
-                color="primary"
-                appearance="outline"
-                [hasIcon]="true"
-                [text]="copy().labels.apply"
-              />
-            </a>
+            <app-link-button
+              class="landing-card__button-link"
+              size="small"
+              appearance="outline"
+              [href]="path"
+              [text]="copy().labels.apply"
+            />
           } @else {
             <pi-action-button
               size="small"