ui(alerts): drop 'Severity / kind' header — column shows kind only

The text in that column is the kind chip (backup_failed, etc).
Severity is encoded entirely in colour: severity dot, left-border
accent, and the chip's tint. The header promised both but only one
made it into text, which read as if the severity word should
appear too.

Header now reads 'Kind'. Colour does the severity work, as it
already did.

web/templates/pages/alerts.html

@@ -96,7 +96,7 @@
     {{/* header row */}}
     <div class="alert-row head">
       <div></div>
-      <div>Severity / kind</div>
+      <div>Kind</div>
       <div>Host</div>
       <div>Message</div>
       <div>Raised</div>