docs(fix[widgets/mcp-install]): drop tab transition that flickered on theme resolve

The 120ms `transition: color, border-color, background` on
`.lm-mcp-install__tab` animated whenever Furo's CSS variables recomputed —
including the moment gp-sphinx's FOWT script set `body[data-theme]` and
unhid the body. `visibility:hidden` does not pause transitions, so the
in-flight animation tail leaked into view as a brief tab-BG flicker on
initial paint with a saved selection. Same failure mode gp-sphinx fixed on
Furo's `.sig` in 88f7a8b.

Tabs now snap on hover/click, matching that precedent. The prehydrate
machinery in `_prehydrate.py` (669fbc2) was already correct; nothing else
needed to change.

Author: tony

docs/_widgets/mcp-install/widget.css

@@ -37,7 +37,6 @@
   padding: 0.45rem 0.9rem;
   margin: 0;
   white-space: nowrap;
-  transition: color 120ms, border-color 120ms, background 120ms;
 }
 
 .lm-mcp-install__tab:hover {