[Kanti]

Adds Component Snapshot tests and updates to the Frontend Studio backend. Test individual variants, components or folders, or use Test all for the complete catalog, including filtered and collapsed rows. Tests render saved fixtures with the selected preview site and language.

GUI testing is enabled by default. The Enable GUI snapshot testing checkbox in Settings → Extension Configuration → frontend_studio hides the controls and disables backend snapshot requests when cleared. CLI tests and CI remain available. Snapshot requests require a backend login, a valid route token and access to the Frontend Studio module.

Tree actions appear on hover or keyboard focus. Green checkmarks require every variant in the scope to pass; red failure buttons and ✕ Results (N) open the details. The toolbar shows the passed/total count. Failure details include site/language context, a full-height variant list that scrolls independently, and a word diff with two surrounding HTML lines where available. Should and Got retain source line numbers; dynamic values use the same masking as the CLI. Guidance appears above the diff. Go to Component preserves the preview context, Copy Snapshot file location copies the project-relative path, and Copy details includes context and output. HTML and stack traces are escaped.

Normal GUI and CLI tests never write snapshot files. Missing snapshots show a warning, their file location and an explicit Create snapshot action. The GUI creates only the displayed variant's baseline for the selected site and language; CLI --update / -u creates or updates the selected scope. New files report yellow CREATED, and creating or changing files returns a nonzero CLI exit code. Review and commit the file, then rerun without updating. Created and updated snapshots do not count as passed.

Update snapshot replaces only the selected variant's existing baseline for the displayed context. Failed creation or updates retain the original result and any mismatch diff, display the write error separately, and keep the retry action available. Production and its subcontexts allow comparisons but block snapshot creation and updates.

Starting a run immediately clears previous results and discovery errors within its scope. Partial runs preserve unrelated results and errors. Site/language or component-file changes clear the catalog and results, cancel pending requests, and ignore stale responses. The toolbar shows 0/0 passed until discovery rebuilds the catalog. Snapshot files are excluded from component discovery and the development watcher.

Closes #26.