refactor: replace any types with concrete types

[abimbolaalabi]

Overview

This PR replaces the remaining any type annotations with concrete types (or unknown plus narrowing), so external data entry points stay type-checked. A few test mocks and helpers fell back to any, disabling checking exactly where assumptions are made.

While verifying the checks required by this issue, npm run build exposed a pre-existing Next.js build break on main (the StreamDetail named export in a route file), so this PR also fixes that so the typecheck and build gates both pass.

Related Issue

Closes #155

Changes

• [MODIFY] components/header.test.tsx — useState: (initial: any) → (initial: unknown).
• [MODIFY] app/streams/[id]/page.test.tsx — useState: (initial: any) → (initial: unknown); next/link mock children: any → children: ReactNode.
• [MODIFY] components/skeleton.test.tsx — type the Suspense render trees with concrete ReactElement<...> shapes (fixes the pre-existing typecheck failures from calling components directly under React 19 types).
• [ADD] components/stream-detail.tsx — StreamDetail (+ its Field helper) moved out of the route file.
• [MODIFY] app/streams/[id]/page.tsx — drop the StreamDetail named export (Next.js route files may only export route-specific symbols), import it from the new component.
• [MODIFY] components/skeleton.tsx — update the StreamDetailSkeleton comment to the new module location.

Verification Results

npm run lint
npm run typecheck
npm test
npm run build

• npm run lint — passes (only 2 pre-existing warnings in lib/api-schema.test.ts).
• npm run typecheck — passes cleanly (was failing on the pre-existing skeleton.test.tsx errors).
• npm run build — passes, all 6 routes prerender (was failing on main due to the StreamDetail route-file export).
• npm test — unchanged from baseline (same pre-existing environmental failures; no new failures).

Acceptance Criteria	Status
Remaining any types are replaced with concrete types or unknown plus narrowing	✅ 0 remaining any annotations
The typecheck passes	✅ tsc --noEmit clean
No runtime behaviour changes	✅ Purely type-level changes; StreamDetail rendering unchanged

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