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.

stores/settings-store.ts

@@ -220,6 +220,15 @@ interface SettingsState {
   // configured, in which case the view defaults to all non-special (no-role)
   // folders of the active account.
   enableAllMailView: boolean;
+
+  // Cross-account "All accounts" views (gated per-view by the admin policy)
+  enableCrossUnreadView: boolean;
+  enableCrossStarredView: boolean;
+  enableCrossAllView: boolean;
+
+  // Folder ids merged into the virtual "All Mail" mailbox. `null` = never
+  // configured, in which case the view defaults to all non-special (no-role)
+  // folders of the active account.
   allMailFolderIds: string[] | null;
 
   // Email Display
@@ -409,6 +418,10 @@ const DEFAULT_SETTINGS = {
   enableAllMailView: false,
   allMailFolderIds: null as string[] | null,
 
+  enableCrossUnreadView: false,
+  enableCrossStarredView: false,
+  enableCrossAllView: false,
+
   // Email Display
   disableThreading: false,
 
@@ -583,6 +596,9 @@ export const useSettingsStore = create<SettingsState>()(
           includeGroupInUnified: state.includeGroupInUnified,
           enableAllMailView: state.enableAllMailView,
           allMailFolderIds: state.allMailFolderIds,
+          enableCrossUnreadView: state.enableCrossUnreadView,
+          enableCrossStarredView: state.enableCrossStarredView,
+          enableCrossAllView: state.enableCrossAllView,
           senderFavicons: state.senderFavicons,
           showAvatarsInJunk: state.showAvatarsInJunk,
           colorfulSidebarIcons: state.colorfulSidebarIcons,