style: stack per-class override rows instead of a wide table (REQ-181 layout)

The width:100% table plus the new MaxBox column let the Class column absorb
the slack, stranding the Container checkbox away from its class. One block
per class now: name + Container checkbox on the first line, Conf/IoU/MinBox/
MaxBox inputs in a wrapping grid below. Behavior identical - KEYS,
buildClassParams, compose() copy line, toggleContainer revert, aria labels.

docs/design.md

@@ -359,9 +359,11 @@ that class to its full-set detections (blank if no full-set run covered it); cle
 examples is then a plain full-set view again. Run Preview always refreshes the full-set for
 every selected class first (with empty exemplars), then re-runs the exemplared classes
 sequentially; drawing an example never triggers a full-set re-run. The
-per-class overrides table also carries a **Container** checkbox (REQ-184) that toggles
+per-class overrides block (REQ-181) stacks one row per class — name plus **Container**
+checkbox on the first line, the four override inputs in a wrapping grid below — and that
+checkbox toggles
 `project_classes.container` through `PATCH /api/projects/{id} { containers: … }`. The same
-table carries a **Copy** button (REQ-186): one click puts every selected class's effective
+block carries a **Copy** button (REQ-186): one click puts every selected class's effective
 settings (per-class override where set, else the global slider; container = the checkbox
 state) on the clipboard as short lines `class <name> conf … iou … minbox … maxbox … container
 true|false`, through `clipboard.js`'s `copyText` (Clipboard API with an `execCommand`