No browser chosen, no "Open on" button

The plain address is the one thing that button must not offer -- followed
in the browser the rewrite rules are installed in, it comes straight back
here -- so there is nothing to show until there is a browser to hand it to.
The copy button and the selectable URL were always the part carrying the
weight; the open link now ships hidden and the script reveals it along with
the scheme.

That also settles what the button should say. It went back to "Open on
<platform>": the browser's name was there to explain a tap that went
nowhere, and a button that is not shown until it works needs no such
explanation.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01KF5YF3iZVKbezwALap8LYd

src/render/challenge.ts

@@ -28,7 +28,7 @@ export function renderChallenge(challenge: Challenge, box: Box | undefined): str
     <footer class="post__foot">
       <div class="original">
         <button type="button" class="copy" id="give-up">Give up and show me the link</button>
-        <a class="original__open" href="${challenge.originalUrl}" rel="noopener noreferrer nofollow" target="_blank">
+        <a class="original__open" href="${challenge.originalUrl}" rel="noopener noreferrer nofollow" target="_blank" hidden>
           Open on ${challenge.platformLabel}
         </a>
         <p class="original__url"><code>${challenge.originalUrl}</code></p>