Add issue creation and editing

TESTING.md

@@ -141,6 +141,21 @@ xcrun simctl launch booted de.rfc1437.gotcha
       selections persist independently per repository and status persists.
 - [ ] The issue-filter icon uses a neutral color for Open + All Milestones + no
       labels, and the app accent color whenever any non-default filter is set.
+- [ ] Tap the add button. **New Issue** appears as a native modal with Cancel
+      and Save, title and Markdown body fields, multi-select labels, a
+      single-select optional milestone, and an optional inline due-date picker.
+- [ ] In the new-issue body, enter headings, emphasis, a list, a link, and a
+      fenced code block. **Write** syntax-highlights the Markdown and **Preview**
+      renders it; switching repeatedly preserves the exact source text.
+- [ ] Save is disabled for an empty or whitespace-only title. Create issues with
+      no labels/milestone/due date and with multiple labels, a milestone, and a
+      due date; each new issue appears in the list and opens its detail.
+- [ ] Tap **Edit** on an issue. Change its title and Markdown body, replace and
+      clear labels, select and clear its milestone, and add, change, and remove
+      its due date. Save, refresh, and relaunch to verify every change persists.
+- [ ] Cancel both new and edited issues, including with the keyboard and date
+      picker visible. Nothing is saved, the modal dismisses normally, and the
+      issue list/detail remains usable with Dynamic Type and VoiceOver labels.
 - [ ] Open both an open and a closed issue. A green open or purple closed icon
       appears beside the title and VoiceOver announces the state; author
       metadata, Markdown body, and comments remain correct. Links and