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).

locales/ru/common.json

@@ -114,6 +114,7 @@
     "unified_archive": "Все архивы",
     "unified_junk": "Весь спам",
     "all_accounts": "Все аккаунты",
+    "unified_mailbox": "Общий почтовый ящик",
     "expand": "Развернуть",
     "collapse": "Свернуть",
     "expand_tooltip": "Развернуть",
@@ -924,14 +925,16 @@
       "unified_mailbox": {
         "label": "Общий почтовый ящик",
         "description": "Показывать объединённые папки (Входящие, Отправленные и др.) для всех подключённых аккаунтов",
+        "cross_account": {
+          "label": "Across all accounts",
+          "description": "Merge the unified mailbox across every connected account instead of staying within the active account."
+        },
         "include_group": {
           "label": "Включать групповые ящики",
           "description": "Также объединять общие/групповые ящики в едином представлении."
         }
       },
       "all_mail": {
-        "label": "All Mail",
-        "description": "Show an \"All Mail\" entry above your folders that merges messages from across this account's folders into one list.",
         "folders_label": "Folders in All Mail",
         "folders_description": "Choose which folders are merged into the All Mail view.",
         "account_hint": "Applies to {account}.",