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/email/email-context-menu.tsx

@@ -307,7 +307,7 @@ export function EmailContextMenu({
 
       {/* Move to submenu */}
       {moveTree.length > 0 && (
-        <ContextMenuSubMenu icon={FolderInput} label={t("move_to")}>
+        <ContextMenuSubMenu icon={FolderInput} label={t("move_to")} testId="ctx-move-to">
           {(() => {
             const renderNodes = (nodes: MailboxNode[]) => {
               return nodes.map((node) => {
@@ -320,6 +320,7 @@ export function EmailContextMenu({
                       <ContextMenuItem
                         icon={Icon}
                         label={nodeLabel}
+                        testId={`move-to:${node.id}`}
                         onClick={() =>
                           handleAction(() =>
                             showBatchActions