fix(webdav): fully working WebDAV RFC 4918 implementation

- Move WebDAV routes to top-level (out of /api nest) for proper path handling
- Add trailing slash routes and HEAD method support
- Refactor all 12 handlers to use Axum State extractor instead of req.extensions()
- Fix MOVE handler to support rename (same-folder move) via rename_file service
- Add Overwrite header support in MOVE/COPY operations
- Add extract_webdav_path() helper for consistent path parsing
- Add precondition_failed variant to AppError
- All 17 integration tests passing: OPTIONS, PROPFIND, MKCOL, PUT, GET, HEAD,
  PROPPATCH, COPY, MOVE, LOCK, DELETE (files and folders)

src/interfaces/api/routes.rs

@@ -276,11 +276,8 @@ pub fn create_api_routes(app_state: &AppState) -> Router<AppState> {
         tracing::warn!("Trash service not available - trash view will not work");
     }
     
-    // Add WebDAV routes
-    {
-        use crate::interfaces::api::handlers::webdav_handler;
-        router = router.merge(webdav_handler::webdav_routes());
-    }
+    // NOTE: WebDAV routes are mounted at top-level (/webdav) in main.rs
+    // for client compatibility, NOT under /api.
     
     // NOTE: CalDAV and CardDAV routes are mounted at top-level (/caldav, /carddav)
     // in main.rs for protocol compliance, NOT under /api.