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-hover-actions.tsx

@@ -4,7 +4,7 @@ import { Email } from "@/lib/jmap/types";
 import { useSettingsStore } from "@/stores/settings-store";
 import type { HoverAction } from "@/stores/settings-store";
 import { cn } from "@/lib/utils";
-import { Trash2, Star, Mail, MailOpen, Archive, Tag, ShieldAlert } from "lucide-react";
+import { Trash2, Star, Mail, MailOpen, Archive, Tag, ShieldAlert, ShieldCheck } from "lucide-react";
 import { useTranslations } from "next-intl";
 import { useIsMobile } from "@/hooks/use-media-query";
 
@@ -17,6 +17,10 @@ interface EmailHoverActionsProps {
   onArchive?: () => void;
   onSetColorTag?: (color: string | null) => void;
   onMarkAsSpam?: () => void;
+  // When the email lives in a junk folder (incl. the aggregate "All Junk" view)
+  // the spam quick-action flips to "not spam".
+  isInJunk?: boolean;
+  onUndoSpam?: () => void;
 }
 
 const ACTION_CONFIG: Record<HoverAction, {
@@ -72,6 +76,8 @@ export function EmailHoverActions({
   onArchive,
   onSetColorTag,
   onMarkAsSpam,
+  isInJunk = false,
+  onUndoSpam,
 }: EmailHoverActionsProps) {
   const hoverActions = useSettingsStore((state) => state.hoverActions);
   const hoverActionsMode = useSettingsStore((state) => state.hoverActionsMode);
@@ -106,7 +112,8 @@ export function EmailHoverActions({
         onSetColorTag?.(null);
         break;
       case "spam":
-        onMarkAsSpam?.();
+        if (isInJunk) onUndoSpam?.();
+        else onMarkAsSpam?.();
         break;
     }
   };
@@ -116,20 +123,28 @@ export function EmailHoverActions({
     if (!config) return null;
     const Icon = config.icon;
 
+    // In a junk context the spam action becomes "not spam".
+    const isNotSpam = actionId === "spam" && isInJunk;
     const DisplayIcon = actionId === "markRead"
       ? (isUnread ? MailOpen : Mail)
       : actionId === "star" && isStarred
         ? Star
-        : Icon;
+        : isNotSpam
+          ? ShieldCheck
+          : Icon;
+    const title = isNotSpam ? t("not_spam") : t(config.titleKey);
+    const className = isNotSpam
+      ? "hover:text-green-600 dark:hover:text-green-400"
+      : config.className;
 
     return (
       <button
         key={actionId}
         onClick={(e) => handleAction(e, actionId)}
-        title={t(config.titleKey)}
+        title={title}
         className={cn(
           "p-1.5 rounded-md transition-colors duration-100 text-muted-foreground hover:bg-black/5 dark:hover:bg-white/10",
-          config.className,
+          className,
         )}
       >
         <DisplayIcon