fix(nc): enable Nextcloud Android app connectivity and uploads

- Add /remote.php/dav discovery endpoint for Android app server detection
- Add /index.php/204 connectivity check endpoint (returns 204 No Content)
- Redirect login flow to nc:// deep link for mobile credential delivery
- Support GET/HEAD on folders (NC clients use as existence checks)
- Recursive MKCOL to create missing parent directories
- Fix single-file PROPFIND returning empty multistatus response
- Strip instance suffix from preview fileId (e.g. "00000326ocnca")
- Add recommendations stub endpoint

src/interfaces/api/handlers/auth_handler.rs

@@ -621,7 +621,11 @@ async fn oidc_callback(
                     user = %username,
                     "OIDC login completed Nextcloud Login Flow v2 successfully"
                 );
-                Ok(Redirect::temporary("/nextcloud-success.html"))
+                let nc_url = format!(
+                    "nc://login/server:{}&user:{}&password:{}",
+                    base_url, username, app_password
+                );
+                Ok(Redirect::temporary(&nc_url))
             } else {
                 tracing::error!(
                     user = %username,