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/nl/common.json

@@ -114,6 +114,7 @@
     "unified_archive": "Alle archieven",
     "unified_junk": "Alle spam",
     "all_accounts": "Alle accounts",
+    "unified_mailbox": "Gecombineerd postvak",
     "expand": "Uitklappen",
     "collapse": "Inklappen",
     "expand_tooltip": "Uitklappen",
@@ -924,14 +925,16 @@
       "unified_mailbox": {
         "label": "Gecombineerd postvak",
         "description": "Gecombineerde mappen (Postvak IN, Verzonden, enz.) van alle verbonden accounts weergeven",
+        "cross_account": {
+          "label": "Across all accounts",
+          "description": "Merge the unified mailbox across every connected account instead of staying within the active account."
+        },
         "include_group": {
           "label": "Groepspostvakken meenemen",
           "description": "Gedeelde/groepspostvakken ook in de gecombineerde weergave opnemen."
         }
       },
       "all_mail": {
-        "label": "All Mail",
-        "description": "Show an \"All Mail\" entry above your folders that merges messages from across this account's folders into one list.",
         "folders_label": "Folders in All Mail",
         "folders_description": "Choose which folders are merged into the All Mail view.",
         "account_hint": "Applies to {account}.",