feat(unified-mailbox): account-bounded Unified Mailbox with opt-in cross-account

Rework the sidebar "All accounts" section into a "Unified Mailbox" that, by
default, stays within the active login account and its shared/group folders.
Merging across multiple logged-in accounts becomes an opt-in sub-option instead
of the default, and the standalone per-account "All Mail" virtual folder is
folded into the unified All mail / Unread / Starred entries (its folder selection
now narrows those lists).

Scope:
- lib/unified-mailbox.ts: UnifiedAccountClient.crossIncludedMailboxIds; the cross
  views honor the per-account folder selection (union across accounts = the sum
  of each account's selection), falling back to inbox+custom when unset.
- stores/email-store.ts: buildUnifiedAccountClients gains scopeToClientAccountId
  (the account boundary) and populates crossIncludedMailboxIds from
  allMailFolderIds; remove the standalone __all_mail__ fetch/search/load-more
  branches.
- page.tsx: scope to the active account unless cross-account is active (per-user
  opt-in AND admin gate); the per-role unified mailboxes obey the same scope.

Folding:
- Drop ALL_MAIL_MAILBOX_ID (lib/jmap/types.ts); thread-list source-folder column
  now keys on isUnifiedView only; settings folder picker moves under the unified
  group and shows once any unified entry is enabled.

Config:
- User: new unifiedCrossAccount (default false); includeGroupInUnified default
  flips to true; enableAllMailView retired; the three cross-view toggles now gate
  the unified Unread/Starred/All mail entries.
- Admin: new unifiedCrossAccountEnabled gate, default FALSE (cross-account is an
  admin opt-in; when off the per-user toggle is hidden and the scope is forced
  account-bounded at runtime). allMailViewEnabled deprecated and normalized
  forward into crossAllViewEnabled on policy load; cross-view gate labels reworded
  to "Unified Mailbox: ...".

Header: the sidebar section shows "All accounts" when cross-account is active
(opt-in AND admin gate AND >1 connected account), else "Unified Mailbox".

Migration:
- Settings persist v5 -> v6 (exported migrateSettings) - cross-active users keep
  cross-account; All-Mail-only users get the account-bounded unified All mail
  entry with folder ids preserved; includeGroupInUnified enabled for every
  migrated config; fresh installs are account-bounded.
- Admin policy: one-shot, marker-guarded migratePolicyUnifiedMailbox (run before
  configManager.load) enables unifiedCrossAccountEnabled when a cross view was
  active, so existing cross-account installs keep the behaviour despite the
  default-false gate. Skipped on read-only config dirs.

Locales: sidebar all_accounts (original label) + unified_mailbox (translated, per
locale) keys; dead standalone all_mail strings removed across all 20 locales.

Docs: FEATURES.md updated to the account-bounded model, the cross-account gate,
and the folder-narrowed aggregate entries.

Verification: tsc clean, eslint clean, full vitest suite green (incl. translations
completeness, cross-view/migration coverage, and the admin policy migration test).

components/email/thread-list-item.tsx

@@ -2,7 +2,7 @@
 
 import React, { useCallback } from "react";
 import { formatDate, formatDateTime, stripInvisibleLeading } from "@/lib/utils";
-import { Email, ThreadGroup, ALL_MAIL_MAILBOX_ID } from "@/lib/jmap/types";
+import { Email, ThreadGroup } from "@/lib/jmap/types";
 import { cn } from "@/lib/utils";
 import { SelectableAvatar } from "@/components/email/selectable-avatar";
 import { Paperclip, Star, Pin, Circle, ChevronRight, ChevronDown, Loader2, MessageSquare, CheckSquare, Square, Reply, Forward, CalendarClock, Folder } from "lucide-react";
@@ -97,7 +97,7 @@ const SingleEmailItem = React.forwardRef<HTMLDivElement, SingleEmailItemProps>(
     const showAvatarsInJunk = useSettingsStore((state) => state.showAvatarsInJunk);
     const hideJunkAvatarImages = currentMailboxRole === 'junk' && !showAvatarsInJunk;
     // Show the originating folder in the aggregate "All …" views.
-    const showSourceFolder = (isUnifiedView || selectedMailbox === ALL_MAIL_MAILBOX_ID) && !!email.sourceFolder;
+    const showSourceFolder = isUnifiedView && !!email.sourceFolder;
     const getAccountById = useAccountStore((state) => state.getAccountById);
     const accountColor = email.accountId ? getAccountById(email.accountId)?.avatarColor : undefined;
     const isChecked = selectedEmailIds.has(email.id);
@@ -459,7 +459,7 @@ export const ThreadListItem = React.forwardRef<HTMLDivElement, ThreadListItemPro
       : null;
 
     const { selectedMailbox, mailboxes, selectedEmailIds, toggleEmailSelection, selectRangeEmails, clearSelection, isUnifiedView, unifiedRole } = useEmailStore();
-    const showSourceFolder = (isUnifiedView || selectedMailbox === ALL_MAIL_MAILBOX_ID) && !!latestEmail.sourceFolder;
+    const showSourceFolder = isUnifiedView && !!latestEmail.sourceFolder;
     const getAccountById = useAccountStore((state) => state.getAccountById);
     const threadAccountColor = latestEmail.accountId ? getAccountById(latestEmail.accountId)?.avatarColor : undefined;
     // In Sent/Drafts folders, show recipient instead of sender (which is always