Allow milestones to be closed and reopened

TESTING.md

@@ -303,13 +303,16 @@ xcrun simctl launch booted de.rfc1437.gotcha
 - [ ] Open a milestone and verify every assigned issue and pull request is
       listed; tapping either opens its normal detail.
 - [ ] Tap the add button. **New Milestone** has native Cancel and Save controls,
-      title and multiline description fields, and an optional inline date
-      picker. Save is disabled for an empty or whitespace-only title. Create
-      milestones both without and with a due date and verify each appears.
+      title and multiline description fields, a **Closed** switch that defaults
+      off, and an optional inline date picker. Save is disabled for an empty or
+      whitespace-only title. Create milestones both without and with a due date
+      and verify each appears open.
 - [ ] Tap the pencil button on a milestone. Change its title and description, add and
-      change its due date, then turn Due Date off and save to clear it. Refresh
-      and relaunch after each save to verify persistence; Cancel leaves every
-      value unchanged. Exercise the form with the keyboard, Dynamic Type, and
+      change its due date, then turn Due Date off and save to clear it. Turn
+      **Closed** on and save; verify the list and detail show the closed state.
+      Edit it again, turn **Closed** off, and verify it reopens. Refresh and
+      relaunch after each save to verify persistence; Cancel leaves every value
+      unchanged. Exercise the form with the keyboard, Dynamic Type, and
       VoiceOver labels.
 - [ ] Repository issue lists and issue details show the assigned milestone with
       a flag icon instead of a text label.