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/messages/de.json

@@ -318,7 +318,8 @@
       "port": "Port",
       "encryption": "Verschluesselung",
       "encryptionNone": "Keine",
-      "folder": "Ordner",
+      "folder": "Ordner / Postfach",
+      "folderExchangeHelp": "Standard-Postfaecher: Inbox, SentItems, DeletedItems, Drafts, JunkEmail",
       "senderFilter": "Absenderfilter",
       "pollInterval": "Abrufintervall",
       "pollIntervalSuffix": "Minuten",

apps/web/src/messages/en.json

@@ -318,7 +318,8 @@
       "port": "Port",
       "encryption": "Encryption",
       "encryptionNone": "None",
-      "folder": "Folder",
+      "folder": "Folder / Mailbox",
+      "folderExchangeHelp": "Common mailboxes: Inbox, SentItems, DeletedItems, Drafts, JunkEmail",
       "senderFilter": "Sender Filter",
       "pollInterval": "Poll Interval",
       "pollIntervalSuffix": "Minutes",