test(integration): draft handling and shared-folder moves

Add draft and shared-folder-move coverage (suite now 31 tests). Findings are
asserted server-side or pinned with test.fail where the UI is incomplete.

Drafts (07):
- multiple recipients (committed and typed-but-uncommitted) persist, and the
  draft reopens via the continue-draft button;
- a server-created draft (with $draft) shows the continue-draft button;
- a changed sender identity is saved to the draft on the server;
- KNOWN BUG (test.fail): reopening a draft resets the From selector to the
  default identity instead of the one the draft was saved with.

Shared-folder moves (08):
- shared -> shared (same owner) moves work in both directions (server-verified);
- KNOWN LIMITATION (test.fail): cross-account moves (own account <-> shared
  folder) don't relocate the message — the Move-to submenu offers the target
  but clicking it is a no-op.

Hooks added: composer From select + save-status, viewer edit-draft button,
context-menu "Move to" submenu + per-target testids (testId on
ContextMenuSubMenu). Helpers: JMAP identities/createDraft/sharing, composer
drive + move-via-submenu. README documents the findings.

components/ui/context-menu.tsx

@@ -157,12 +157,15 @@ interface ContextMenuSubMenuProps {
   icon?: React.ComponentType<{ className?: string }>;
   label: string;
   children: React.ReactNode;
+  /** Stable hook for integration tests (not user-visible). */
+  testId?: string;
 }
 
 export function ContextMenuSubMenu({
   icon: Icon,
   label,
   children,
+  testId,
 }: ContextMenuSubMenuProps) {
   const [isOpen, setIsOpen] = useState(false);
   const [subMenuPos, setSubMenuPos] = useState<Position | null>(null);
@@ -236,6 +239,7 @@ export function ContextMenuSubMenu({
         role="menuitem"
         aria-haspopup="true"
         aria-expanded={isOpen}
+        data-testid={testId}
       >
         {Icon && <Icon className="w-4 h-4 flex-shrink-0" />}
         <span className="flex-1">{label}</span>