FN-5913: fix dashboard spinner anchoring

Keep dashboard loading spinners rotating around the correct center.

- switch shared SVG spinner utilities from fill-box to view-box anchoring
- factor spinner CSS assertions into a shared contract helper
- add lucide Loader2 coverage and a regression test against fill-box anchoring

Files changed:
 .../app/__tests__/spinner-animation.css.test.ts    | 50 ++++++++++++++--------
 packages/dashboard/app/styles.css                  |  2 +-
 2 files changed, 34 insertions(+), 18 deletions(-)

Fusion-Task-Id: FN-5913

Fusion-Task-Lineage: 5fbf2577-301e-4813-ac15-30a73267d155

packages/dashboard/app/styles.css

@@ -197,7 +197,7 @@ html {
 
 svg.animate-spin,
 svg.spin {
-  transform-box: fill-box;
+  transform-box: view-box;
 }
 
 :root {