fix(ui): the issue composer opens on its title, not on the Edit switch

A form dialog opens on the body's first control, and the composer's first
control is the Edit / Preview switch above the form: the focus landed there,
at open and when a retried load brought the form, and typing went nowhere. A
form can now mark the field to open on (data-initial-focus); every other
dialog keeps its first control. The Modal's comment on the re-armed watch now
says where the focus goes when the focused control is removed but others stay.

frontend/src/components/ui/dialogFocus.ts

@@ -16,6 +16,15 @@ const FOCUSABLE =
 export const focusablesIn = (root: HTMLElement): HTMLElement[] =>
   [...root.querySelectorAll<HTMLElement>(FOCUSABLE)].filter((el) => el.offsetParent !== null);
 
+/** A form's first field — where a dialog opened to fill something in takes the focus: its first
+ *  control, unless one is marked `data-initial-focus`. The mark is for a form with a control above
+ *  its fields that is not one of them — the issue composer's Edit / Preview switch took the focus,
+ *  and typing went nowhere (E32 review). */
+export const firstFieldIn = (root: HTMLElement): HTMLElement | undefined => {
+  const all = focusablesIn(root);
+  return all.find((el) => el.hasAttribute("data-initial-focus")) ?? all[0];
+};
+
 /** Keep Tab / Shift+Tab cycling inside `scope` instead of reaching the inert page behind the
  *  backdrop. Call from a keydown handler that has already decided this dialog is the topmost layer —
  *  a nested dialog must defer to its child. `container` is the scope's own tabIndex=-1 panel, if it