fix(ui): draw session lineage lines in blue for contrast

[Ark0N]

Follow-up to #285, owner request: "also make these lines in blue that they are better visible".

Violet sits close to the terminal's own dim foreground, so the arcs lost contrast exactly where they cross text, which is most of their length. Blue reads at a glance on the dark skins and on the light ones.

• Colour still comes from a token every skin block already defines and tunes for its own background (--session-blue in place of --session-purple), so it stays one rule for all seven skins with no per-skin override, and the two blues are not even the same: --session-blue is per palette while the subagent rule hardcodes #3b82f6.
• Hue no longer separates this layer from the subagent lines, so the separation now rests entirely on shape (a lineage arc hangs under the strip and never reaches a window), weight and dash pattern. Written into the rule so it does not get eroded later.

CSS only: no geometry, no markup, no settings. Verified at 1:1 in the same harness #285 used (real styles.css + real computeLineagePath()), across the adjacent, far and wrapped-strip layouts, on a dark skin and a light one.