feat(unified-mailbox): account-bounded Unified Mailbox with opt-in cross-account

Rework the sidebar "All accounts" section into a "Unified Mailbox" that, by
default, stays within the active login account and its shared/group folders.
Merging across multiple logged-in accounts becomes an opt-in sub-option instead
of the default, and the standalone per-account "All Mail" virtual folder is
folded into the unified All mail / Unread / Starred entries (its folder selection
now narrows those lists).

Scope:
- lib/unified-mailbox.ts: UnifiedAccountClient.crossIncludedMailboxIds; the cross
  views honor the per-account folder selection (union across accounts = the sum
  of each account's selection), falling back to inbox+custom when unset.
- stores/email-store.ts: buildUnifiedAccountClients gains scopeToClientAccountId
  (the account boundary) and populates crossIncludedMailboxIds from
  allMailFolderIds; remove the standalone __all_mail__ fetch/search/load-more
  branches.
- page.tsx: scope to the active account unless cross-account is active (per-user
  opt-in AND admin gate); the per-role unified mailboxes obey the same scope.

Folding:
- Drop ALL_MAIL_MAILBOX_ID (lib/jmap/types.ts); thread-list source-folder column
  now keys on isUnifiedView only; settings folder picker moves under the unified
  group and shows once any unified entry is enabled.

Config:
- User: new unifiedCrossAccount (default false); includeGroupInUnified default
  flips to true; enableAllMailView retired; the three cross-view toggles now gate
  the unified Unread/Starred/All mail entries.
- Admin: new unifiedCrossAccountEnabled gate, default FALSE (cross-account is an
  admin opt-in; when off the per-user toggle is hidden and the scope is forced
  account-bounded at runtime). allMailViewEnabled deprecated and normalized
  forward into crossAllViewEnabled on policy load; cross-view gate labels reworded
  to "Unified Mailbox: ...".

Header: the sidebar section shows "All accounts" when cross-account is active
(opt-in AND admin gate AND >1 connected account), else "Unified Mailbox".

Migration:
- Settings persist v5 -> v6 (exported migrateSettings) - cross-active users keep
  cross-account; All-Mail-only users get the account-bounded unified All mail
  entry with folder ids preserved; includeGroupInUnified enabled for every
  migrated config; fresh installs are account-bounded.
- Admin policy: one-shot, marker-guarded migratePolicyUnifiedMailbox (run before
  configManager.load) enables unifiedCrossAccountEnabled when a cross view was
  active, so existing cross-account installs keep the behaviour despite the
  default-false gate. Skipped on read-only config dirs.

Locales: sidebar all_accounts (original label) + unified_mailbox (translated, per
locale) keys; dead standalone all_mail strings removed across all 20 locales.

Docs: FEATURES.md updated to the account-bounded model, the cross-account gate,
and the folder-narrowed aggregate entries.

Verification: tsc clean, eslint clean, full vitest suite green (incl. translations
completeness, cross-view/migration coverage, and the admin policy migration test).

lib/admin/config-manager.ts

@@ -33,12 +33,12 @@ class ConfigManager {
     this.adminConfig = await this.readJsonFile('config.json') || {};
     const policy = await this.readJsonFile('policy.json');
     if (policy) {
-      this.policyCache = {
+      this.policyCache = ConfigManager.normalizePolicy({
         ...DEFAULT_POLICY,
         ...policy,
         features: { ...DEFAULT_FEATURE_GATES, ...(policy.features || {}) },
         themePolicy: { ...DEFAULT_THEME_POLICY, ...(policy.themePolicy || {}) },
-      };
+      });
     } else {
       this.policyCache = { ...DEFAULT_POLICY };
     }
@@ -166,15 +166,28 @@ class ConfigManager {
    */
   async setPolicy(policy: SettingsPolicy): Promise<void> {
     assertWritable('update settings policy');
-    this.policyCache = {
+    this.policyCache = ConfigManager.normalizePolicy({
       ...DEFAULT_POLICY,
       ...policy,
       features: { ...DEFAULT_FEATURE_GATES, ...(policy.features || {}) },
       themePolicy: { ...DEFAULT_THEME_POLICY, ...(policy.themePolicy || {}) },
-    };
+    });
     await this.writeJsonFile('policy.json', this.policyCache as unknown as Record<string, unknown>);
   }
 
+  /**
+   * Migrates deprecated feature gates forward. The standalone "All Mail" view
+   * (`allMailViewEnabled`) was folded into the unified "All mail" entry, so an
+   * admin who enabled it keeps that entry available via `crossAllViewEnabled`.
+   * Idempotent - safe to run on every load.
+   */
+  private static normalizePolicy(policy: SettingsPolicy): SettingsPolicy {
+    if (policy.features.allMailViewEnabled) {
+      policy.features.crossAllViewEnabled = true;
+    }
+    return policy;
+  }
+
   /**
    * Reload config from disk (for manual file edits or multi-instance).
    */