feat(PROJ-33): IMAP UID-Stabilität + Shared/Personal-Modus

Backend:
- storage: uid BIGSERIAL Migration, MailWithUID, GetMailsWithUID, GetMailsByRecipient
- tenantstore: imap_mode Spalte, GetIMAPMode, SetIMAPMode
- imapserver: stable UIDs aus DB, personal/shared Modus, userEmail in session
- api: GET/PUT /api/admin/settings/imap-mode (domain_admin only, double opt-in)

Frontend:
- IMAPSettingsTab: Modus-Anzeige + Toggle mit Double-Opt-In Dialog
- Admin-Panel: IMAP-Tab für domain_admin

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/admin/page.tsx

@@ -75,6 +75,7 @@ import { TenantsTab } from "@/components/admin/tabs/TenantsTab";
 import { LabelsTab } from "@/components/admin/tabs/LabelsTab";
 import { CertTab } from "@/components/admin/tabs/CertTab";
 import { ModulesTab } from "@/components/admin/ModulesTab";
+import { IMAPSettingsTab } from "@/components/admin/tabs/IMAPSettingsTab";
 import { ResetPasswordDialog, DeleteUserDialog } from "@/components/admin/UserDialogs";
 
 const AUDIT_PAGE_SIZE = 25;
@@ -799,6 +800,9 @@ export default function AdminPage() {
             {!isSuperAdmin && user?.role === "domain_admin" && (
               <TabsTrigger value="tenant-ldap" onClick={loadTenantLDAP}>LDAP</TabsTrigger>
             )}
+            {!isSuperAdmin && user?.role === "domain_admin" && (
+              <TabsTrigger value="imap-settings">IMAP</TabsTrigger>
+            )}
             {isSuperAdmin && <TabsTrigger value="labels" onClick={loadLabelsTab}>Labels</TabsTrigger>}
             {isSuperAdmin && <TabsTrigger value="security">Security</TabsTrigger>}
             {isSuperAdmin && <TabsTrigger value="cert" onClick={loadCert}>Zertifikat</TabsTrigger>}
@@ -1075,6 +1079,12 @@ export default function AdminPage() {
             </TabsContent>
           )}
 
+          {!isSuperAdmin && (
+            <TabsContent value="imap-settings">
+              <IMAPSettingsTab />
+            </TabsContent>
+          )}
+
           <TabsContent value="modules" className="mt-4">
             <ModulesTab />
           </TabsContent>