feat: add dual-layer versioning to WebDAV sync (protocol v2 + db-v6)

Separate protocol version from database compatibility version in WebDAV
sync paths. Upload writes to v2/db-v6/<profile>, download falls back to
legacy v2/<profile> when current path has no data. Extend manifest with
optional dbCompatVersion field and add legacy layout detection to UI.

src-tauri/src/services/webdav.rs

@@ -477,7 +477,8 @@ mod tests {
             "https://dav.example.com/remote.php/dav/files/demo/",
             &[
                 "cc switch-sync".to_string(),
-                "v3".to_string(),
+                "v2".to_string(),
+                "db-v6".to_string(),
                 "default profile".to_string(),
                 "manifest.json".to_string(),
             ],
@@ -485,7 +486,7 @@ mod tests {
         .unwrap();
         assert_eq!(
             url,
-            "https://dav.example.com/remote.php/dav/files/demo/cc%20switch-sync/v3/default%20profile/manifest.json"
+            "https://dav.example.com/remote.php/dav/files/demo/cc%20switch-sync/v2/db-v6/default%20profile/manifest.json"
         );
         assert!(!url.contains("//cc"), "should not have double-slash");
     }