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).

lib/jmap/types.ts

@@ -892,19 +892,12 @@ export function isUnifiedMailboxId(id: string): boolean {
 }
 
 /**
- * Virtual mailbox id for the gated "All Mail" view: every folder of a single
- * account merged into one date-sorted list. Distinct from the unified mailbox
- * ids above, which merge one role across multiple accounts. Which folders are
- * included is a per-user setting (see `allMailFolderIds`).
- */
-export const ALL_MAIL_MAILBOX_ID = '__all_mail__';
-
-/**
- * Cross-account "All …" views shown in the unified ("All accounts") section.
- * Each merges messages across EVERY account (including shared/group folders),
- * spanning all folders except junk/spam, sent, archive, trash and drafts, in
- * one date-sorted list. Distinct from the per-role unified ids (one role across
- * accounts) and from ALL_MAIL_MAILBOX_ID (all folders of a single account).
+ * Cross views shown in the unified ("Unified Mailbox") section: All mail /
+ * Unread / Starred. Each merges messages across the account boundary (the active
+ * account + its shared folders by default, or every logged-in account when the
+ * cross-account sub-option is on), narrowed by the user's folder selection (see
+ * `allMailFolderIds`). Distinct from the per-role unified ids (one role across
+ * accounts).
  */
 export const CROSS_UNREAD = '__cross_unread__';
 export const CROSS_STARRED = '__cross_starred__';