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.service.ts

@@ -175,7 +175,7 @@ export class DkvService {
    *
    * T-05-13: Decrypted password used only within this method scope — never logged.
    */
-  async testConnection(tenantId: string, dto: DkvConfigDto): Promise<boolean> {
+  async testConnection(tenantId: string, dto: DkvConfigDto): Promise<{ success: boolean; message?: string }> {
     let password: string | undefined = dto.password;
 
     // If no password in DTO, fall back to the stored one