React migration phase 2d: user profile page

[vibhaseshadri-cognition]

Summary

Ports the Angular UserComponent (src/app/user/*) to React, replacing the web/src/pages/UserPage.tsx placeholder. DOM structure, class names and copy are kept identical to the Angular template.

Behaviour mapping:

• route.params.subscribe(...) → useEffect keyed on useParams().id, re-fetching (and clearing user/errorMessage) when the id changes, with a cancelled flag so a stale response can't overwrite newer state.
• rendering follows the Angular *ngIf chain exactly: !user && !errorMessage → <Loader />, !user && errorMessage !== '' → <ErrorMessage message={Could not load user ${id}.} />, otherwise the .profile markup.
• [innerHTML]="user.about" → dangerouslySetInnerHTML, only rendered when about is set.
• Location.back() → useNavigate()(-1) on the .back-button span.

web/src/user/user.scss is the component SCSS verbatim with imports repointed at web/src/styles/; the Angular-only :host >>> pre deep selector becomes .profile pre (same effect, about HTML is rendered inside .profile).

Tests (web/src/user/UserPage.test.tsx) cover the loader→profile transition, the full profile markup, a user without about, the error state, and the back button (asserted via real MemoryRouter history, not a mocked useNavigate).

npm run lint, npm test (32 passing) and npm run build all pass. Base branch is the phase 1 branch, not master.

Rendered locally against a stubbed API response:

Link to Devin session: https://app.devin.ai/sessions/a13b52a5d39c4bea9eef18845250b691
Requested by: @vibhaseshadri-cognition

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