fix(themes): use --sf-color-action-source-dark in dark selection-bg

The [data-theme="dark"] override for --sf-color-selection-bg was still
referencing --sf-color-action-source-light. This override takes precedence
over the light-dark() fix in tokens.css, so dark-mode selection highlights
were drawing from the light source (oklch 0.5 0.22 235) instead of the
darker auto-derived source (oklch 0.7 0.198 235). Update to -source-dark
for semantic correctness and to match the P8 baseline.

Author: claude

core/themes.css

@@ -223,7 +223,7 @@
       --sf-color-link--visited: oklch(from var(--sf-color-action) clamp(0.68, l, 1) c calc(h + 60));
 
       /* Selection */
-      --sf-color-selection-bg: oklch(from var(--sf-color-action-source-light) clamp(0.62, calc(0.93 - l * 0.4), 0.78) c h / 0.55);
+      --sf-color-selection-bg: oklch(from var(--sf-color-action-source-dark) clamp(0.62, calc(0.93 - l * 0.4), 0.78) c h / 0.55);
 
       /* Status — strong variants (dark: push lightness up) */
       --sf-color-success-strong: oklch(from var(--sf-color-success) clamp(0.70, calc(l + 0.15), 1) c h);