Zone page: links to the same zone in the other views

The header says "also in VIEW" for every zone of the same name in
another view, linked or not, in the view's colour, keeping the open tab
where the other zone has it. The zone API gains in_views.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -8,6 +8,10 @@ All notable changes to Jarl are documented here. The format follows
 
 ### Added
 
+- A zone's page links to the same zone in the other views ("also in
+  external"), in each view's colour, keeping the tab you are on. It
+  works for zones that aren't linked too, and the zone API's new
+  `in_views` lists them.
 - `jarl.service` has the alias `jarld.service`, so `systemctl restart
   jarld` works too. The alias is linked when the unit is enabled; the
   package relinks a unit enabled by an older version on upgrade.