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/Modal.tsx

@@ -14,16 +14,17 @@ import { X } from "lucide-react";
 import { useBackToClose } from "../../lib/history";
 import { anyBlockingLayerAbove, pushLayer, removeLayer, takeEscape } from "../../lib/layerStack";
 import { useFullscreenPortalHost } from "../../lib/fullscreen";
-import { focusablesIn, trapTab, useDialogChrome } from "./dialogFocus";
+import { firstFieldIn, focusablesIn, trapTab, useDialogChrome } from "./dialogFocus";
 import { IconButton } from "./IconButton";
 import Toaster from "./Toaster";
 import { useOverlayOrder } from "./overlayOrder";
 import { SURFACE } from "./surfaces";
 
-/** Where a dialog opens focused (C4, by purpose — WAI-ARIA APG): `field` = the body's first control
- *  (a form), `close` = the header's ✕ (anything to read or browse), `action` = the child that takes
- *  `autoFocus` itself (a confirm's least destructive button; the ✕ if none does). Required, so every
- *  dialog states its purpose instead of inheriting "the first focusable", which is always the ✕. */
+/** Where a dialog opens focused (C4, by purpose — WAI-ARIA APG): `field` = the body's first control,
+ *  or the one marked `data-initial-focus` (a form), `close` = the header's ✕ (anything to read or
+ *  browse), `action` = the child that takes `autoFocus` itself (a confirm's least destructive button;
+ *  the ✕ if none does). Required, so every dialog states its purpose instead of inheriting "the first
+ *  focusable", which is always the ✕. */
 export type ModalFocus = "field" | "close" | "action";
 
 // The footer slot of the open dialog. `ModalFooter` portals into it from any depth, so a body that
@@ -100,18 +101,19 @@ export default function Modal({
     // Only a rendered target (the trap's own `offsetParent` test); none falls back to the hook's
     // first-focusable rule. `action` lands on the ✕ here and its child's autoFocus then takes over.
     initial: () =>
-      (initialFocus === "field" && bodyRef.current ? focusablesIn(bodyRef.current)[0] : null) ??
+      (initialFocus === "field" && bodyRef.current ? firstFieldIn(bodyRef.current) : null) ??
       (closeRef.current?.offsetParent != null ? closeRef.current : null),
   });
   // A form that loads after the dialog opens (the issue composer shows a spinner first) has no field
-  // to take the focus at open, so it lands on the ✕. When the first control arrives the focus moves
-  // there — but only while it still rests on the ✕: a user who has moved it keeps it where it is.
+  // to take the focus at open, so it lands on the ✕. When the form arrives the focus moves to its
+  // first field — but only while it still rests on the ✕: a user who has moved it keeps it there.
   // The form takes it ONCE: arrived while the focus was elsewhere, it disarms all the same, or a later
   // mutation would pull the focus off a ✕ the user went back to (E32 review round 2). The watch
   // re-arms only when the focus is taken away, not moved: the body has no control again, or the
   // focused one went (a failed load's Retry giving way to the spinner, or straight to the form) and
-  // the focus fell to <body> — it then waits on the ✕ for the form (E32 review). Only <body>: focus
-  // in a dialog stacked on top is not this one's to take.
+  // the focus fell to <body>. It then goes to the ✕, and on to the first field as soon as the body
+  // has one — at once, when other controls stayed beside the one that went (E32 review). Only
+  // <body>: focus in a dialog stacked on top is not this one's to take.
   useEffect(() => {
     const body = bodyRef.current;
     if (initialFocus !== "field" || !body || focusablesIn(body).length > 0) return;
@@ -121,7 +123,7 @@ export default function Modal({
         closeRef.current.focus();
         armed = true;
       }
-      const first = focusablesIn(body)[0];
+      const first = firstFieldIn(body);
       if (!first) {
         armed = true;
         return;