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).

stores/__tests__/settings-store-all-mail.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, beforeEach } from 'vitest';
-import { useSettingsStore } from '../settings-store';
+import { useSettingsStore, migrateSettings } from '../settings-store';
 
 describe('settings-store per-account allMailFolderIds', () => {
   beforeEach(() => {
@@ -53,3 +53,43 @@ describe('settings-store per-account allMailFolderIds', () => {
     });
   });
 });
+
+describe('migrateSettings v5 -> v6 (Unified Mailbox rework)', () => {
+  it('keeps cross-account users cross-account when any cross view was on, and enables shared', () => {
+    const out = migrateSettings(
+      { allMailFolderIds: {}, enableCrossUnreadView: true, enableAllMailView: false, includeGroupInUnified: false },
+      5,
+    ) as unknown as Record<string, unknown>;
+    expect(out.unifiedCrossAccount).toBe(true);
+    // shared inclusion is enabled for every migrated config, even if it was off
+    expect(out.includeGroupInUnified).toBe(true);
+    expect(out.enableAllMailView).toBeUndefined();
+  });
+
+  it('folds a standalone All-Mail user into the account-bounded unified "All mail" entry', () => {
+    const out = migrateSettings(
+      {
+        allMailFolderIds: { 'acct-1': ['inbox', 'projects'] },
+        enableAllMailView: true,
+        enableUnifiedMailbox: false,
+        enableCrossUnreadView: false,
+        enableCrossStarredView: false,
+        enableCrossAllView: false,
+      },
+      5,
+    ) as unknown as Record<string, unknown>;
+    expect(out.enableUnifiedMailbox).toBe(true);
+    expect(out.enableCrossAllView).toBe(true);
+    expect(out.unifiedCrossAccount).toBe(false);   // new account-bounded default
+    expect(out.includeGroupInUnified).toBe(true);
+    // folder selection carries over unchanged -> narrows the unified lists
+    expect(out.allMailFolderIds).toEqual({ 'acct-1': ['inbox', 'projects'] });
+    expect(out.enableAllMailView).toBeUndefined();
+  });
+
+  it('a fresh user gets account-bounded defaults', () => {
+    const out = migrateSettings({ allMailFolderIds: {} }, 5) as unknown as Record<string, unknown>;
+    expect(out.unifiedCrossAccount).toBe(false);
+    expect(out.includeGroupInUnified).toBe(true);
+  });
+});