feat(settings): make Node & Security columns responsive (stack when narrow)

The NODE + SECURITY split was a fixed 60/40 two-column layout regardless of
width, which cramps the RPC grid and wastes space on narrow windows. Make it
responsive: below a configurable content width (node-sec-stack-width, default
600) the two columns stack into one full-width column — SECURITY flows below
NODE — so each section gets the full width; side-by-side 60/40 is kept when
there's room. Column-geometry-only change (leftX/rightX/widths + the right
column's top Y); the content blocks are untouched. The rejoin already used
max(leftBottom, rightBottom), correct in both modes.

Full-node + lite build clean; source hygiene clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

res/themes/ui.toml

@@ -1325,6 +1325,9 @@ wallet-btn-padding = { size = 24.0 }
 rpc-label-min-width = { size = 70.0 }
 rpc-label-width = { size = 85.0 }
 security-combo-width = { size = 120.0 }
+# Node & Security: below this card-content width the NODE + SECURITY columns stack
+# vertically (full width each) instead of sitting side by side.
+node-sec-stack-width = { size = 600.0 }
 port-input-min-width = { size = 60.0 }
 port-input-width-ratio = { size = 0.4 }
 idle-combo-width = { size = 64.0 }