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/ja/common.json

@@ -134,7 +134,10 @@
     "nav_label": "ナビゲーション",
     "add_app": "アプリ",
     "shared": "共有",
-    "scheduled": "予約済み"
+    "scheduled": "予約済み",
+    "unified_all_unread": "すべての未読",
+    "unified_all_starred": "すべてのスター付き",
+    "unified_all_mail": "すべてのメール"
   },
   "protocol_handlers": {
     "title": "既定のアプリ",
@@ -930,6 +933,18 @@
         "description": "デスクトップ専用のパワーユーザー向けレイアウトで、マルチタブのメッセージ閲覧やアカウント横断のワークフローに対応します。標準インターフェイスには影響せず、いつでも元に戻せます。",
         "open_label": "Pro インターフェイスを開く",
         "back_to_standard": "標準に戻る"
+      },
+      "cross_unread": {
+        "label": "すべてのアカウント: 未読",
+        "description": "「すべてのアカウント」セクションに、すべてのアカウントの未読メールを一覧表示する項目を表示します。迷惑メール、送信済み、アーカイブ、ゴミ箱、下書きを除くすべてのフォルダーが対象です。"
+      },
+      "cross_starred": {
+        "label": "すべてのアカウント: スター付き",
+        "description": "「すべてのアカウント」セクションに、すべてのアカウントのスター付きメールを一覧表示する項目を表示します。迷惑メール、送信済み、アーカイブ、ゴミ箱、下書きを除くすべてのフォルダーが対象です。"
+      },
+      "cross_all": {
+        "label": "すべてのアカウント: すべてのメール",
+        "description": "「すべてのアカウント」セクションに、すべてのアカウントのすべてのメールを一覧表示する項目を表示します。迷惑メール、送信済み、アーカイブ、ゴミ箱、下書きを除くすべてのフォルダーが対象です。"
       }
     },
     "keywords": {
@@ -1186,6 +1201,7 @@
         "archive": "アーカイブ",
         "tag": "ラベル",
         "spam": "スパムとしてマーク",
+        "not_spam": "迷惑メールではない",
         "none_selected": "アクションが選択されていません",
         "mode_label": "表示モード",
         "mode_inline": "インライン",