Quarterly recurrence is a common real-world cadence (dentist, oil change, etc.)
but was missing from the interval dropdown.

Assisted-by: Claude Opus 4.6 via pi

diff --git a/lib/views/series/new.rb b/lib/views/series/new.rb
index e87f363..da76783 100644
--- a/lib/views/series/new.rb
+++ b/lib/views/series/new.rb
@@ -33,6 +33,7 @@ module Views
                     option(value: "day") { "day(s)" }
                     option(value: "week") { "week(s)" }
                     option(value: "month") { "month(s)" }
+                    option(value: "quarter") { "quarter(s)" }
                     option(value: "year") { "year(s)" }
                   end
                 end