Document Windows read-only WebDAV quirks and add locking opt-out

The Explorer optimistically removes icons on a rejected DELETE, and apps
that LOCK before open get stuck for 30-60s on the WebClient negative
cache when the user only has read access. New disable-webdav-locking.reg
turns SupportLocking off as an opt-in workaround for view-only setups.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

windows/disable-webdav-locking.reg

@@ -0,0 +1,17 @@
+Windows Registry Editor Version 5.00
+
+; Disables WebDAV file locking globally. Useful when staff users have
+; only "read" rights on customer folders: Office/editors stop trying
+; to LOCK the file before opening (which would 403 and trigger the
+; "file does not exist, create new?" dialog). Read-only viewing then
+; works without the artificial wait.
+;
+; Trade-off: collaborative editing scenarios cannot rely on WebDAV
+; locks anymore. For pure download/view workflows this is fine.
+;
+; Rechtsklick -> "Zusammenführen" (als Administrator), dann
+; den WebClient-Dienst neu starten:
+;     net stop WebClient && net start WebClient
+
+[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WebClient\Parameters]
+"SupportLocking"=dword:00000000