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.

locales/ko/common.json

@@ -134,7 +134,10 @@
     "nav_label": "내비게이션",
     "add_app": "앱",
     "shared": "공유됨",
-    "scheduled": "예약됨"
+    "scheduled": "예약됨",
+    "unified_all_unread": "모든 읽지 않음",
+    "unified_all_starred": "모든 별표",
+    "unified_all_mail": "모든 메일"
   },
   "protocol_handlers": {
     "title": "기본 앱",
@@ -930,6 +933,18 @@
         "description": "데스크톱 전용 파워 유저 레이아웃으로, 다중 탭 메시지 탐색과 계정 간 워크플로우를 지원합니다. 표준 인터페이스에는 영향이 없으며 언제든지 되돌릴 수 있습니다.",
         "open_label": "Pro 인터페이스 열기",
         "back_to_standard": "표준으로 돌아가기"
+      },
+      "cross_unread": {
+        "label": "모든 계정: 읽지 않음",
+        "description": "모든 계정 섹션에 모든 계정의 읽지 않은 메일을 나열하는 항목을 표시합니다. 스팸, 보낸편지함, 보관함, 휴지통, 임시보관함을 제외한 모든 폴더가 대상입니다."
+      },
+      "cross_starred": {
+        "label": "모든 계정: 별표",
+        "description": "모든 계정 섹션에 모든 계정의 별표 메일을 나열하는 항목을 표시합니다. 스팸, 보낸편지함, 보관함, 휴지통, 임시보관함을 제외한 모든 폴더가 대상입니다."
+      },
+      "cross_all": {
+        "label": "모든 계정: 모든 메일",
+        "description": "모든 계정 섹션에 모든 계정의 모든 메일을 나열하는 항목을 표시합니다. 스팸, 보낸편지함, 보관함, 휴지통, 임시보관함을 제외한 모든 폴더가 대상입니다."
       }
     },
     "keywords": {
@@ -1186,6 +1201,7 @@
         "archive": "보관",
         "tag": "태그",
         "spam": "스팸으로 표시",
+        "not_spam": "스팸 아님",
         "none_selected": "선택된 액션 없음",
         "mode_label": "표시 모드",
         "mode_inline": "목록 안쪽",