fix(07): propagate Exchange connection error, add folder selection for Exchange

- testConnection now returns { success, message? } instead of boolean so
  admins see the actual EWS/IMAP error in the UI rather than "Unbekannter Fehler"
- Exchange provider: resolveFolder() maps folder string to WellKnownFolderName
  (Inbox, SentItems, DeletedItems, Drafts, JunkEmail + German aliases)
- InboxConfigForm: folder field now shown for both IMAP and Exchange protocols
  with Exchange-specific help text listing valid well-known names
- Controller returns testConnection result directly (no more redundant wrapping)

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

apps/web/src/app/(portal)/modules/dkv-fleet/settings/components/InboxConfigForm.tsx

@@ -303,22 +303,23 @@ export function InboxConfigForm() {
         </div>
       )}
 
-      {/* IMAP-only: Ordner */}
-      {form.protocol === 'imap' && (
-        <div>
-          <label htmlFor="dkv-folder" className={labelCls}>
-            {t('form.folder')} *
-          </label>
-          <input
-            id="dkv-folder"
-            type="text"
-            required
-            className={inputCls}
-            value={form.folder}
-            onChange={(e) => update('folder', e.target.value)}
-          />
-        </div>
-      )}
+      {/* Ordner / Postfach */}
+      <div>
+        <label htmlFor="dkv-folder" className={labelCls}>
+          {t('form.folder')}
+        </label>
+        <input
+          id="dkv-folder"
+          type="text"
+          placeholder={form.protocol === 'exchange' ? 'Inbox' : 'INBOX'}
+          className={inputCls}
+          value={form.folder}
+          onChange={(e) => update('folder', e.target.value)}
+        />
+        {form.protocol === 'exchange' && (
+          <p className="mt-1 text-xs text-muted-foreground">{t('form.folderExchangeHelp')}</p>
+        )}
+      </div>
 
       {/* Exchange-only: Domain */}
       {form.protocol === 'exchange' && (