Upgrade TypeScript from 4.6 to 5.x

[talissoncosta]

Problem

The frontend uses TypeScript 4.6.4 (released March 2022). Newer tooling increasingly assumes >= 4.9 or 5.x, and staying on 4.6 creates growing friction.

Risks of staying on 4.6

• Missing language features — satisfies (4.9), const type parameters (5.0), decorators (5.0), using declarations (5.2) are all unavailable. Contributors familiar with modern TS will hit friction.
• Weaker type inference — each TS release tightens inference, narrowing, and control flow analysis. Staying on 4.6 means we miss bug-catching improvements the compiler could give us for free.
• Security and bug fixes — TS 4.6 is no longer maintained. No patches for compiler bugs or security issues.
• Ecosystem drift — @types/* packages and libraries increasingly assume >= 4.9 or 5.x. We already see this with @sinclair/typebox (Storybook dep) failing typecheck because it uses syntax introduced after 4.6.
• Storybook upgrade blocked — upgrading to Storybook 10+ requires TypeScript >= 4.9. We reverted to Storybook 8 (chore: design system colour tokens, primitives & Storybook #6883) specifically because of this constraint.

Scope estimate

A trial upgrade to 4.9 surfaced ~805 type errors, mostly from stricter checks. Upgrading to 5.x will likely surface more. This should be treated as a dedicated effort, not bundled with other work.

Proposed fix

Upgrade TypeScript to the latest 5.x (currently 5.9.3) and fix any type errors introduced by stricter checks.

Acceptance criteria

• TypeScript upgraded to latest 5.x
• npm run typecheck passes
• npm ci works without --legacy-peer-deps
• CI green

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Part of the Design System Audit (#6606)

[talissoncosta]

It has already been handled on #6417