Fix genuine React anti-patterns flagged by lint

[dqnykamp]

When enabling the ESLint CI job, several pre-existing lint violations were silenced with eslint-disable comments. Three of these are genuine issues worth fixing properly:

1. setState inside useMemo — src/Viewer/Viewer.tsx

setErrMsg is called inside a useMemo callback as error handling for invalid activity source. This is a React anti-pattern that can cause infinite render loops. The error derivation should be restructured — for example, derive the error inline during render (outside useMemo) and use useMemo only for the pure computation.

Rule: react-hooks/set-state-in-render

2. Ref read/write during render — src/activity-viewer.tsx

lastPropSet.current is both read and written during render to track prop changes for variant index generation. Accessing/mutating refs during render is a React anti-pattern. This should be replaced with a useEffect or restructured to avoid the render-time ref mutation entirely.

Rule: react-hooks/refs

3. Untyped spy access in tests — src/test/activityStateReducer.test.ts

spy.mock.lastCall![0].message_id is accessed without typing lastCall[0], requiring file-level disables for no-unsafe-assignment, no-unsafe-member-access, and no-non-null-assertion. Casting to the appropriate message type (e.g., ReportStateMessage) would fix all three.

Rule: @typescript-eslint/no-unsafe-assignment, no-unsafe-member-access, no-non-null-assertion