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/settings/layout-settings.tsx

@@ -118,12 +118,18 @@ function MailLayoutPreview({
 export function LayoutSettings() {
   const t = useTranslations('settings.appearance');
   const tEmail = useTranslations('settings.email_behavior');
-  const { toolbarPosition, showToolbarLabels, hideAccountSwitcher, showRailAccountList, enableUnifiedMailbox, includeGroupInUnified, enableAllMailView, allMailFolderIds, colorfulSidebarIcons, mailLayout, proInterface, updateSetting } = useSettingsStore();
+  const { toolbarPosition, showToolbarLabels, hideAccountSwitcher, showRailAccountList, enableUnifiedMailbox, includeGroupInUnified, enableAllMailView, allMailFolderIds, enableCrossUnreadView, enableCrossStarredView, enableCrossAllView, colorfulSidebarIcons, mailLayout, proInterface, updateSetting } = useSettingsStore();
   const { isSettingLocked, isSettingHidden, isFeatureEnabled } = usePolicyStore();
   const accounts = useAccountStore(s => s.accounts);
   const mailboxes = useEmailStore(s => s.mailboxes);
   const hasGroupInboxes = useMemo(() => mailboxes.some(m => m.isShared), [mailboxes]);
   const allMailViewAllowed = isFeatureEnabled('allMailViewEnabled');
+  // Cross-account "All accounts" views, each gated independently by the admin.
+  const crossViews = [
+    { setting: 'enableCrossUnreadView', value: enableCrossUnreadView, allowed: isFeatureEnabled('crossUnreadViewEnabled'), labelKey: 'cross_unread.label', descKey: 'cross_unread.description' },
+    { setting: 'enableCrossStarredView', value: enableCrossStarredView, allowed: isFeatureEnabled('crossStarredViewEnabled'), labelKey: 'cross_starred.label', descKey: 'cross_starred.description' },
+    { setting: 'enableCrossAllView', value: enableCrossAllView, allowed: isFeatureEnabled('crossAllViewEnabled'), labelKey: 'cross_all.label', descKey: 'cross_all.description' },
+  ] as const;
 
   // Own (non-shared) folders and the current All Mail selection. `null` =
   // never configured, which defaults to all non-special (no-role) folders.
@@ -226,6 +232,26 @@ export function LayoutSettings() {
         </div>
       )}
 
+      {enableUnifiedMailbox && crossViews.some(c => c.allowed) && (
+        <div className="ml-4 border-l-2 border-border pl-4 -mt-2 space-y-2">
+          {crossViews.map(({ setting, value, allowed, labelKey, descKey }) => (
+            allowed && !isSettingHidden(setting) && (
+              <SettingItem
+                key={setting}
+                label={t(labelKey)}
+                description={t(descKey)}
+                locked={isSettingLocked(setting)}
+              >
+                <ToggleSwitch
+                  checked={value}
+                  onChange={(v) => updateSetting(setting, v)}
+                />
+              </SettingItem>
+            )
+          ))}
+        </div>
+      )}
+
       {allMailViewAllowed && !isSettingHidden('enableAllMailView') && (
         <SettingItem
           label={t('all_mail.label')}