fix(ui): give <small> a deterministic font-size so hints stay consistent

The base small rule set --app-font-size: 0.875em but never consumed it,
so <small> elements fell back to the browser default ("smaller", roughly
0.83em of the parent). That made hints under fields visually drift
depending on the parent context — most noticeably on the telemetry page
where the muted hint, the per-field hint inside a label, and the caption
under a fieldset all rendered slightly different sizes.

The codebase already standardises on var(--text-xs) wherever a small was
explicitly styled (gridjs, footer, docs); this commit just makes the
default match that convention so every bare <small> is 12px regardless
of where it sits.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/css/layout/app-shell.css

@@ -11,7 +11,7 @@
   }
 
   small {
-    --app-font-size: 0.875em;
+    font-size: var(--text-xs);
     line-height: var(--leading-loose);
   }