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/api/src/dkv/dkv.controller.ts

@@ -103,8 +103,7 @@ export class DkvController {
   @Roles(Role.ADMIN, Role.SUPER_ADMIN)
   async testConnection(@Req() req: any, @Body() dto: DkvConfigDto) {
     const tenantId = this._requireTenant(req);
-    const success = await this.dkvService.testConnection(tenantId, dto);
-    return { success };
+    return this.dkvService.testConnection(tenantId, dto);
   }
 
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