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).

app/(main)/admin/_tabs/policy.tsx

@@ -6,7 +6,9 @@ import type { SettingsPolicy, FeatureGates } from '@/lib/admin/types';
 import { DEFAULT_FEATURE_GATES, DEFAULT_POLICY } from '@/lib/admin/types';
 import { apiFetch } from '@/lib/browser-navigation';
 
-const EXCLUDED_FEATURE_GATES: (keyof FeatureGates)[] = ['pluginsEnabled', 'pluginsUploadEnabled', 'themesEnabled', 'userThemesEnabled'];
+// `allMailViewEnabled` is deprecated (folded into `crossAllViewEnabled`, normalized
+// forward on policy load), so it is hidden from the admin UI.
+const EXCLUDED_FEATURE_GATES: (keyof FeatureGates)[] = ['pluginsEnabled', 'pluginsUploadEnabled', 'themesEnabled', 'userThemesEnabled', 'allMailViewEnabled'];
 
 const FEATURE_GATE_LABELS: Partial<Record<keyof FeatureGates, { label: string; description: string }>> = {
   sidebarAppsEnabled: { label: 'Sidebar Apps', description: 'Allow custom web apps in navigation rail' },
@@ -22,10 +24,10 @@ const FEATURE_GATE_LABELS: Partial<Record<keyof FeatureGates, { label: string; d
   folderIconsEnabled: { label: 'Folder Icons', description: 'Allow custom folder icon picker' },
   hoverActionsConfigEnabled: { label: 'Hover Actions Config', description: 'Allow users to customize email hover actions' },
   filesEnabled: { label: 'Files (WebDAV)', description: 'Enable file storage via WebDAV. WARNING: Large uploads can cause Stalwart/RocksDB instability. Not recommended for production.' },
-  allMailViewEnabled: { label: 'All Mail View', description: 'Show a virtual "All Mail" folder that merges messages from across an account’s folders into one list. Users choose which folders are included. Requires the per-user toggle in Settings → Appearance.' },
-  crossUnreadViewEnabled: { label: 'All Accounts: Unread', description: 'Allow an "All unread" entry in the All accounts section that lists unread mail across every account (incl. shared folders), spanning all folders except junk, sent, archive, trash and drafts. Requires the matching per-user toggle in Settings → Appearance.' },
-  crossStarredViewEnabled: { label: 'All Accounts: Starred', description: 'Allow an "All starred" entry in the All accounts section that lists flagged/starred mail across every account (incl. shared folders), spanning all folders except junk, sent, archive, trash and drafts. Requires the matching per-user toggle in Settings → Appearance.' },
-  crossAllViewEnabled: { label: 'All Accounts: All Mail', description: 'Allow an "All mail" entry in the All accounts section that lists all mail across every account (incl. shared folders), spanning all folders except junk, sent, archive, trash and drafts. Requires the matching per-user toggle in Settings → Appearance.' },
+  crossUnreadViewEnabled: { label: 'Unified Mailbox: Unread', description: 'Allow an "Unread" entry in the Unified Mailbox section that lists unread mail across the account and its shared folders (or every account when the cross-account sub-option is on). Honors the user\'s folder selection. Requires the matching per-user toggle in Settings → Appearance.' },
+  crossStarredViewEnabled: { label: 'Unified Mailbox: Starred', description: 'Allow a "Starred" entry in the Unified Mailbox section that lists flagged/starred mail across the account and its shared folders (or every account when the cross-account sub-option is on). Honors the user\'s folder selection. Requires the matching per-user toggle in Settings → Appearance.' },
+  crossAllViewEnabled: { label: 'Unified Mailbox: All Mail', description: 'Allow an "All mail" entry in the Unified Mailbox section that lists all mail across the account and its shared folders (or every account when the cross-account sub-option is on). Honors the user\'s folder selection. Requires the matching per-user toggle in Settings → Appearance.' },
+  unifiedCrossAccountEnabled: { label: 'Unified Mailbox: Cross-account', description: 'Allow users to expand the Unified Mailbox beyond the active account boundary so its lists merge across every logged-in account. When off, the Unified Mailbox stays within the active account and its shared folders.' },
 };
 
 const RESTRICTABLE_SETTINGS = [