feat(layout): widen current pane to 50% — PANE_RATIO 2:5:3

Change the parent|current|preview split from 1:2:2 (20/40/40) to
2:5:3 (20/50/30) so the focused list gets more room. 2-pane tabs
now give current the combined 80%. Updates ratio comments and the
PaneRow test expectations.

src/context/navigation-store.ts

@@ -13,7 +13,7 @@
  *
  *        parent  |  current  |  preview
  *
- * Layout ratios (1/5 : 2/5 : 2/5 in the final remake) live in
+ * Layout ratios (20% : 50% : 30% — PANE_RATIO 2:5:3) live in
  * `@/utils/navigation` (PANE_RATIO). This module owns only the *focusable*
  * nav model — which column is focused and where its list cursor lives. The
  * parent/preview columns are always derived, never focused.