Introduce semantic colour tokens and primitive palette

[talissoncosta]

Problem

The frontend has no formal colour system. 280+ hardcoded hex values in TSX files, inverted tonal scales, alpha colour RGB mismatches, and three parallel dark mode mechanisms that don't compose.

Solution

1. Primitive palette — full 50–950 tonal scales for all colour families in _primitives.scss
2. Semantic tokens — CSS custom properties in _tokens.scss organised by role (text, surface, icon, stroke, brand, feedback) with light/dark mode variants
3. TS exports — common/theme/tokens.ts for use in inline styles, automatically responds to theme changes

Progress

• PR chore: design system colour tokens, primitives & Storybook #6883 (chore/design-system-tokens) — primitives, tokens, Storybook stories
• Decouple Button from Redux store (dead feature prop) #6866 — Decouple Button from Redux (blocker for Storybook stories)

Acceptance Criteria

• Semantic tokens cover text, surface, icon, stroke, brand, feedback categories
• Primitive scales defined for all colour families (50–950)
• Light and dark mode values defined for all tokens
• Tokens available as both CSS custom properties and TS exports
• No visual regression — existing UI renders identically

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