UI/UX standards guide: foundations and navigation

**Describe the task**
Establish a shared UI/UX standards guide for the applications this team is modernising, so recurring patterns are built the same way across apps without needing design supervision on every screen. Carbon remains the baseline; the guide records only where we override it or where Carbon leaves a decision open.

**Acceptance Criteria**
- [x] Audit the live apps (FSP, FREP, LEXIS + FDS apps) to establish what is currently implemented (tokens, header, side navigation, typography, colour)
- [x] Agree the documentation model: Carbon as baseline, guide covers only overrides and team decisions, no implementation guidance
- [x] Write the first set of pages: How to use this guide, Colour, Typography, Application header, Side navigation, Return navigation
- [x] Produce supporting visuals for header, side navigation states, selected item, and return patterns
- [x] Record decisions and open items so they survive between sessions
- [x] Publish to Confluence under a top-level "UI Standards" page
- [x] Walk the devs through the guide once published, and fold their feedback back into the pages