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/de/common.json

@@ -134,7 +134,10 @@
     "nav_label": "Navigation",
     "add_app": "Apps",
     "shared": "Geteilt",
-    "scheduled": "Geplant"
+    "scheduled": "Geplant",
+    "unified_all_unread": "Alle ungelesenen",
+    "unified_all_starred": "Alle markierten",
+    "unified_all_mail": "Alle Mails"
   },
   "protocol_handlers": {
     "title": "Standard-Apps",
@@ -930,6 +933,18 @@
         "description": "Desktop-Power-User-Layout mit Multi-Tab-Nachrichtenansicht und kontoübergreifenden Workflows. Die Standardoberfläche bleibt unverändert; Sie können jederzeit zurückwechseln.",
         "open_label": "Pro-Oberfläche öffnen",
         "back_to_standard": "Zurück zum Standard"
+      },
+      "cross_unread": {
+        "label": "Alle Konten: Ungelesen",
+        "description": "Zeigt in der Sektion „All accounts“ einen Eintrag mit ungelesenen Mails über alle Konten, über alle Ordner außer Junk, Gesendet, Archiv, Papierkorb und Entwürfe."
+      },
+      "cross_starred": {
+        "label": "Alle Konten: Markiert",
+        "description": "Zeigt in der Sektion „All accounts“ einen Eintrag mit markierten Mails über alle Konten, über alle Ordner außer Junk, Gesendet, Archiv, Papierkorb und Entwürfe."
+      },
+      "cross_all": {
+        "label": "Alle Konten: Alle Mails",
+        "description": "Zeigt in der Sektion „All accounts“ einen Eintrag mit allen Mails über alle Konten, über alle Ordner außer Junk, Gesendet, Archiv, Papierkorb und Entwürfe."
       }
     },
     "keywords": {
@@ -1186,6 +1201,7 @@
         "archive": "Archivieren",
         "tag": "Label",
         "spam": "Als Spam markieren",
+        "not_spam": "Kein Spam",
         "none_selected": "Keine Aktionen ausgewählt",
         "mode_label": "Anzeigemodus",
         "mode_inline": "Eingebettet",