Make every clickable control look clickable, in two shared styles (closes #375)

Buttons keep btn-primary, btn-secondary and btn-danger and gain a
pointer cursor; the navigation links become btn-secondary buttons.
Every control that was plain coloured text (Copy, both Add, the row
actions, Resubmit, Replay, the back, footer and download links) now
uses btn-small, bordered at rest with hover and focus states. Both
styles are in static/css/style.css, which the layout now loads.

The event log's clickable rows become buttons, so they work by
keyboard; Replay moves beside its delivery's row. Rows on the webhook
page and in the event log wrap at phone width. The browser test now
checks that Copy reads "Copied".

Model: opus-5-5

templates/sources_new.html

@@ -5,7 +5,7 @@
 {{define "content"}}
 <div class="max-w-2xl mx-auto px-6 py-8">
     <div class="mb-6">
-        <a href="/hooks" class="text-sm text-primary-600 hover:text-primary-700">&larr; Back to webhooks</a>
+        <a href="/hooks" class="btn-small">&larr; Back to webhooks</a>
         <h1 class="text-2xl font-medium text-gray-900 mt-2">Create Webhook</h1>
     </div>