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

internal/handlers/ui_copy_test.go

@@ -82,9 +82,9 @@ func TestNavbarUsesWebhookTerminology(t *testing.T) {
 	})
 
 	assert.Contains(t, body, "Retention: 14 days")
-	assert.Contains(t, body, `class="btn-text">Webhooks</a>`)
+	assert.Contains(t, body, `class="btn-secondary">Webhooks</a>`)
 	assert.Contains(
-		t, body, `class="btn-text w-full text-left">Webhooks</a>`,
+		t, body, `class="btn-secondary w-full">Webhooks</a>`,
 	)
 	assert.Contains(
 		t, body,
@@ -163,7 +163,7 @@ func TestEventLogPageIsCalledFullEventLog(t *testing.T) {
 	)
 	assert.Contains(
 		t, detailBody,
-		`<a href="/hook/wh-1/events" class="btn-text text-sm">Full Event Log</a>`,
+		`<a href="/hook/wh-1/events" class="btn-small">Full Event Log</a>`,
 		"the link under recent events",
 	)
 
@@ -331,8 +331,9 @@ func TestEntrypointCopyButtonIsProgressiveEnhancement(t *testing.T) {
 	assert.Contains(t, body, "https://hooks.example.com/h/abc123")
 	assert.Contains(
 		t, body,
-		`hidden data-copy-target="entrypoint-url-ep-1"`,
-		"the button must start hidden and be revealed by script",
+		`<button type="button" hidden data-copy-target="entrypoint-url-ep-1"`,
+		"the copy control must be a button, start hidden and be "+
+			"revealed by script",
 	)
 
 	// renderTemplate streams to the ResponseWriter, so an abort