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/__tests__/unified-mailbox-cross.test.ts

@@ -42,6 +42,26 @@ describe('getCrossIncludedMailboxes', () => {
     const ids = getCrossIncludedMailboxes(account).map((m) => m.id);
     expect(ids).toEqual(['inbox', 'projects']);
   });
+
+  it('honors an explicit crossIncludedMailboxIds selection (folder picker)', () => {
+    const account = makeAccount({
+      accountId: 'a',
+      mailboxes: [mb('inbox', 'inbox'), mb('projects', undefined), mb('archive', 'archive')],
+      // user picked inbox + archive, excluded projects - overrides role exclusion
+      crossIncludedMailboxIds: ['inbox', 'archive'],
+    });
+    const ids = getCrossIncludedMailboxes(account).map((m) => m.id);
+    expect(ids).toEqual(['inbox', 'archive']);
+  });
+
+  it('an empty selection yields no folders', () => {
+    const account = makeAccount({
+      accountId: 'a',
+      mailboxes: [mb('inbox', 'inbox'), mb('projects', undefined)],
+      crossIncludedMailboxIds: [],
+    });
+    expect(getCrossIncludedMailboxes(account)).toEqual([]);
+  });
 });
 
 describe('buildCrossFilter', () => {
@@ -86,6 +106,22 @@ describe('getCrossUnreadTotal', () => {
     });
     expect(getCrossUnreadTotal([a, b])).toBe(10);
   });
+
+  it('counts only the selected folders when crossIncludedMailboxIds is set; shared accounts stay unrestricted', () => {
+    // personal account narrowed to inbox only (projects excluded by the picker)
+    const personal = makeAccount({
+      accountId: 'a',
+      mailboxes: [mb('inbox', 'inbox', 3), mb('projects', undefined, 4)],
+      crossIncludedMailboxIds: ['inbox'],
+    });
+    // shared account unrestricted -> role-exclusion default (inbox + custom)
+    const shared = makeAccount({
+      accountId: 'owner',
+      isShared: true,
+      mailboxes: [mb('ns:inbox', 'inbox', 5, 'orig-inbox'), mb('ns:team', undefined, 2, 'orig-team'), mb('ns:junk', 'junk', 9, 'orig-junk')],
+    });
+    expect(getCrossUnreadTotal([personal, shared])).toBe(3 + 5 + 2);
+  });
 });
 
 describe('resolveSourceFolderName', () => {