feat: cross-account "All accounts" views + full group/shared-account support

Add cross-account aggregate mail views and make group/shared (delegated)
accounts first-class in every aggregate view. (The unified mailbox, the "All
Mail" view, and "include group inboxes" already exist on main; this branch adds
the cross-account views and the shared-account correctness work.)

New views (admin-gated + per-user toggle, nested under Unified Mailbox):
- Cross-account "All accounts": All unread / All starred / All mail across every
  connected account, including shared/group folders. Each list labels the source
  folder of every message.

Source reference on aggregated emails (the core of the shared-account work):
- Replace the overloaded `accountId` with two explicit, always-set fields:
  `sourceClientAccountId` (the login the mail is reachable through) and
  `sourceAccountId` (the owning JMAP account). `accountId` stays display-only.
- Resolution is branch-free everywhere: pick the client by sourceClientAccountId,
  pass sourceAccountId as the JMAP accountId (no-op for personal), read the
  owner's mailbox list cached by JMAP id. No capability scan.

Shared/group-account correctness across all aggregate views:
- Route open (click + auto-fetch), thread/conversation open + reply-refresh,
  mark read, star, move, delete (account-scoped trash), archive (owner-routed
  createMailbox / fetchAccountMailboxes), and spam + undo via the source ref.
- Add accountId params to toggleStar / batchMarkAsRead / batchDeleteEmails /
  createMailbox where missing.
- Fix local unread/total counter math for shared folders via emailInMailbox()
  (matches namespaced shared ids and bare own ids).
- Keep the unified/cross virtual selection on background mailbox refresh (no
  jump back to inbox after deleting in All Drafts/Junk).

Junk UX:
- In "All Junk" the spam action becomes "not spam" in the viewer, context menu,
  and list hover icons; undo routes shared mail back to its own inbox.

Admin:
- Policy gates crossUnread/Starred/AllViewEnabled, each noting the matching
  per-user toggle (allMailViewEnabled clarified too).

i18n / docs / tests:
- locales (19): cross-view labels + descriptions and hover not_spam, translated
  in all shipped languages.
- FEATURES.md + README.md document the new views and group-account support.
- Tests for shared-account routing (single + batch + undoSpam), decoration, and
  unified-selection preservation.

stores/__tests__/folder-management.test.ts

@@ -1,6 +1,7 @@
 import { describe, it, expect, beforeEach, vi } from 'vitest';
 import { useEmailStore } from '../email-store';
 import type { Mailbox } from '@/lib/jmap/types';
+import { UNIFIED_MAILBOX_IDS } from '@/lib/jmap/types';
 
 function makeMailbox(overrides: Partial<Mailbox> = {}): Mailbox {
   return {
@@ -266,4 +267,41 @@ describe('email-store folder management', () => {
       expect(useEmailStore.getState().error).toBe('Role update failed');
     });
   });
+
+  // Regression: a background fetchMailboxes (e.g. push-driven after deleting
+  // drafts in "All Drafts") must not reset a virtual unified/cross-view selection
+  // to the inbox, which would jump the user out of the view they're in.
+  describe('fetchMailboxes selection preservation', () => {
+    it('keeps a unified-view selection (e.g. All Drafts) on background refresh', async () => {
+      useEmailStore.setState({
+        mailboxes: [inbox, sent, trash, custom],
+        selectedMailbox: UNIFIED_MAILBOX_IDS.drafts,
+        isUnifiedView: true,
+      });
+      // Fresh list (not initial load) that does NOT contain the virtual id.
+      const client = makeMockClient({
+        getAllMailboxes: vi.fn().mockResolvedValue([inbox, sent, trash, custom]),
+      });
+
+      await useEmailStore.getState().fetchMailboxes(client);
+
+      expect(useEmailStore.getState().selectedMailbox).toBe(UNIFIED_MAILBOX_IDS.drafts);
+    });
+
+    it('still falls back to inbox when a real selection no longer exists', async () => {
+      useEmailStore.setState({
+        mailboxes: [inbox, sent, trash, custom],
+        selectedMailbox: 'custom-1',
+        isUnifiedView: false,
+      });
+      // custom-1 is gone from the refreshed list.
+      const client = makeMockClient({
+        getAllMailboxes: vi.fn().mockResolvedValue([inbox, sent, trash]),
+      });
+
+      await useEmailStore.getState().fetchMailboxes(client);
+
+      expect(useEmailStore.getState().selectedMailbox).toBe('inbox-1');
+    });
+  });
 });