Merge pull request #509 from hildebrandttk/feat/unified-mailbox-account-scope

Feat/unified mailbox account scope

Rework the sidebar "All accounts" into an account-bounded "Unified Mailbox"
by default, with cross-account merging as an opt-in (admin-gated) sub-option.
The standalone per-account "All Mail" virtual folder is folded into the unified
All mail / Unread / Starred entries.

Conflict resolution notes:
- stores/settings-store.ts: both main and this branch independently added a
  per-account default-identity (#507) migration at different versions (main v6,
  branch v7). Merged migration is version 7 using the refactored migrateSettings
  function; the unified-mailbox rework is guarded at `version < 7` so users who
  stopped at main's interim v6 identity bump still receive it, while the #507
  identity-map coercion stays at `version < 6` so their populated map is kept.
- stores/auth-store.ts: kept main's applyPreferredIdentity (superset with the
  pre-#507 legacy migration).
- stores/email-store.ts: removed the ALL_MAIL_MAILBOX_ID paths (folded into the
  unified views) while preserving main's plugin hooks (onSearchResults /
  onEmailsFetched); adopted advancedSearchCrossViewEmails for advanced cross-view
  search.
- components/settings/layout-settings.tsx: kept main's faviconUnreadBadge setting
  alongside the new unifiedCrossAccount toggle.
- integration/: union-merged the two independently-authored suites - branch suite
  is authoritative (matches new behavior) with main's shared-identity (#569) group
  infrastructure preserved.
- components/email/email-composer.tsx: dropped a duplicate data-testid attribute
  introduced by the auto-merge.

locales/he/common.json

@@ -63,6 +63,7 @@
     "jmap_server_auto_picked": "שרת שנבחר מדומיין הדוא״ל שלך."
   },
   "sidebar": {
+    "unified_mailbox": "תיבת דואר מאוחדת",
     "close": "סגור",
     "compose": "כתיבה",
     "compose_hint": "כתיבה (c)",
@@ -889,14 +890,16 @@
       "unified_mailbox": {
         "label": "תיבת דואר מאוחדת",
         "description": "הצג תיקיות משולבות (דואר נכנס, נשלח וכו׳) בכל החשבונות המחוברים",
+        "cross_account": {
+          "label": "בכל החשבונות",
+          "description": "מזג את תיבת הדואר המאוחדת בכל החשבונות המחוברים במקום להישאר בחשבון הפעיל."
+        },
         "include_group": {
           "label": "כלול תיבות דואר קבוצתיות",
           "description": "גם מזג תיבות דואר משותפות/קבוצתיות לתצוגה המאוחדת."
         }
       },
       "all_mail": {
-        "label": "כל הדוא״ל",
-        "description": "הצג ערך ״כל הדוא״ל״ מעל התיקיות שלך המחברות הודעות מכל תיקיות החשבון לרשימה אחת.",
         "folders_label": "תיקיות בכל הדוא״ל",
         "folders_description": "בחר אילו תיקיות מוזגות לתצוגת כל הדוא״ל.",
         "account_hint": "חל על {account}.",