Fix feature pipeline status dark mode

## Problem

The entire release pipeline visualisation is broken in dark mode. White circles and grey lines render on a dark background, making the component unusable.

### Files

- `web/styles/components/_feature-pipeline-status.scss:2,4,16,20,30,39,43,52`
- Hardcoded: `white` (bg), `#AAA` (border/bg), `#6837fc` (border), `#53af41` (border/bg)

## Solution

Add `.dark` overrides using semantic colour tokens from #6904.

## Blocked by

- #6904 — Semantic colour tokens must land first

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Part of the Design System Audit (#6606) · Epic: #6882