test(integration): dockerized webmail⇆Stalwart Playwright sync suite

Add an end-to-end integration harness that runs the webmail against a real
Stalwart mail server in Docker and drives it with Playwright, focused on the
mail/folder synchronisation behaviour (unread/total counters, folder-list
sync, account-scoped Unified Mailbox) that the unified-mailbox work touches.

- integration/ stack: Stalwart (declarative bootstrap: alice/bob/carol,
  submission + IMAP listeners, permissive CORS) + webmail (dev mode, so the
  browser's plaintext cross-origin JMAP calls aren't blocked by the prod CSP).
- Helpers: dependency-free SMTP submit client, JMAP client for seeding /
  inspecting server state, and page helpers (login, add/switch account,
  locale-independent folder-counter reads).
- Specs: login, single-account sync (receive/read/move/delete/folder-create/
  burst) and multi-account (per-account isolation + cross-account Unified
  Inbox aggregation + background-account delivery). 12 tests, all green.
- Add focused data-testid hooks to the mail UI (folder rows + counters, email
  list items, account switcher, composer) for stable selectors.
- Exclude examples/ and integration/ from tsconfig/eslint/.dockerignore.

components/layout/account-switcher.tsx

@@ -151,6 +151,8 @@ export function AccountSwitcher({ variant = "rail", className }: AccountSwitcher
       <button
         ref={buttonRef}
         onClick={() => setOpen(!open)}
+        data-testid="account-switcher"
+        data-active-account-id={activeAccountId ?? undefined}
         className={cn(
           "flex items-center gap-2 rounded-md transition-colors",
           variant === "rail"
@@ -213,6 +215,9 @@ export function AccountSwitcher({ variant = "rail", className }: AccountSwitcher
                 >
                 <button
                   onClick={() => handleSwitch(account.id)}
+                  data-testid="account-option"
+                  data-account-id={account.id}
+                  data-account-email={account.email || account.username}
                   className={cn(
                     "w-full flex items-start gap-3 px-3 py-2.5 text-start transition-colors",
                     isActive ? "bg-accent/50" : "hover:bg-muted",
@@ -274,6 +279,7 @@ export function AccountSwitcher({ variant = "rail", className }: AccountSwitcher
             <div className="border-t border-border">
               <button
                 onClick={handleAddAccount}
+                data-testid="add-account"
                 className="w-full flex items-center gap-2 px-3 py-2 text-sm text-foreground hover:bg-muted transition-colors"
                 role="menuitem"
               >