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.

components/email/email-list-item.tsx

@@ -29,11 +29,12 @@ interface EmailListItemProps {
   onArchive?: () => void;
   onSetColorTag?: (color: string | null) => void;
   onMarkAsSpam?: () => void;
+  onUndoSpam?: () => void;
 }
 
-export function EmailListItem({ email, selected, onClick, onDoubleClick, onContextMenu, onToggleStar, onMarkAsRead, onDelete, onArchive, onSetColorTag, onMarkAsSpam }: EmailListItemProps) {
+export function EmailListItem({ email, selected, onClick, onDoubleClick, onContextMenu, onToggleStar, onMarkAsRead, onDelete, onArchive, onSetColorTag, onMarkAsSpam, onUndoSpam }: EmailListItemProps) {
   const t = useTranslations('email_viewer');
-  const { selectedEmailIds, toggleEmailSelection, selectRangeEmails, selectedMailbox, mailboxes, clearSelection } = useEmailStore();
+  const { selectedEmailIds, toggleEmailSelection, selectRangeEmails, selectedMailbox, mailboxes, clearSelection, isUnifiedView, unifiedRole } = useEmailStore();
   const showPreview = useSettingsStore((state) => state.showPreview);
   const density = useSettingsStore((state) => state.density);
   const mailLayout = useSettingsStore((state) => state.mailLayout);
@@ -46,8 +47,11 @@ export function EmailListItem({ email, selected, onClick, onDoubleClick, onConte
   const isImportant = email.keywords?.["$important"];
   const isAnswered = email.keywords?.$answered;
   const isForwarded = email.keywords?.$forwarded;
-  // In Sent/Drafts folders, show recipient instead of sender (which is always "me")
-  const currentMailboxRole = mailboxes.find(mb => mb.id === selectedMailbox)?.role;
+  // In Sent/Drafts folders, show recipient instead of sender (which is always "me").
+  // In aggregate role-views the selected mailbox is virtual → fall back to the
+  // unified role so junk-contextual UI (spam ↔ not-spam) and avatar hiding work.
+  const currentMailboxRole = mailboxes.find(mb => mb.id === selectedMailbox)?.role
+    ?? (isUnifiedView ? (unifiedRole ?? undefined) : undefined);
   const showRecipient = currentMailboxRole === 'sent' || currentMailboxRole === 'drafts';
   const sender = showRecipient ? (email.to?.[0] ?? email.from?.[0]) : email.from?.[0];
   const isMobile = useUIStore((state) => state.isMobile);
@@ -321,6 +325,8 @@ export function EmailListItem({ email, selected, onClick, onDoubleClick, onConte
         onArchive={onArchive}
         onSetColorTag={onSetColorTag}
         onMarkAsSpam={onMarkAsSpam}
+        onUndoSpam={onUndoSpam}
+        isInJunk={currentMailboxRole === 'junk'}
       />
     </div>
   );