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/layout/sidebar.tsx

@@ -79,9 +79,10 @@ interface SidebarProps {
   onRefreshMailboxes?: () => void;
   scheduledTotal?: number;
   showScheduledMailbox?: boolean;
-  /** Gated "All Mail" virtual folder that merges all of the account's folders. */
-  showAllMailMailbox?: boolean;
-  /** Gated cross-account views in the "All accounts" section. */
+  /** True when the unified view spans multiple login accounts (cross-account).
+   *  Drives the section header: "All accounts" when true, else "Unified Mailbox". */
+  crossAccountActive?: boolean;
+  /** Gated All mail / Unread / Starred entries in the "Unified Mailbox" section. */
   showCrossUnread?: boolean;
   showCrossStarred?: boolean;
   showCrossAll?: boolean;
@@ -692,7 +693,7 @@ export function Sidebar({
   onRefreshMailboxes,
   scheduledTotal = 0,
   showScheduledMailbox = false,
-  showAllMailMailbox = false,
+  crossAccountActive = false,
   showCrossUnread = false,
   showCrossStarred = false,
   showCrossAll = false,
@@ -1017,20 +1018,10 @@ export function Sidebar({
 
       {/* Mailbox List */}
       <div className="flex-1 overflow-y-auto" data-tour="sidebar">
-        {showAllMailMailbox && (
-          <SidebarRow
-            icon={<Mails className={cn("w-4 h-4 flex-shrink-0", selectedMailbox === '__all_mail__' ? "text-foreground" : "text-muted-foreground")} />}
-            label={t('mailboxes.all_mail')}
-            depth={0}
-            isSelected={!selectedKeyword && selectedMailbox === '__all_mail__'}
-            onClick={() => onMailboxSelect?.('__all_mail__')}
-            isCollapsed={isCollapsed}
-          />
-        )}
         {(showUnified || showCrossUnread || showCrossStarred || showCrossAll) && (
           <div>
             <SidebarSectionHeader
-              label={t("all_accounts")}
+              label={t(crossAccountActive ? "all_accounts" : "unified_mailbox")}
               expanded={unifiedExpanded}
               onToggle={toggleUnified}
               isCollapsed={isCollapsed}