Perf/profile page optimization

[MerlinTheWhiz]

Closes #525

Changes made:

• Converted profile route to a server-rendered shell in src/app/profile/page.tsx:1
• Moved tab interactivity into a small client component with lazy-loaded non-default panels in src/app/profile/components/ProfileTabs.tsx:10
• Split profile/settings/achievements into focused components, with accessible labels, tabs, switches, and memoized panels at src/app/profile/components/ProfileInfoPanel.tsx:1 and src/app/profile/components/SettingsPanel.tsx:1.
• Centralized static data in src/app/profile/profile-data.ts:1.
• Added regression tests in src/app/profile/tests/ProfileTabs.test.tsx:1.
• Added docs in docs/profile-performance.md:1.

Validation:

• Passed: pnpm vitest run src/app/profile/tests/ProfileTabs.test.tsx
• Blocked: pnpm run type-check fails on pre-existing syntax errors in src/components/assessment/AdaptiveTesting.tsx, src/components/assessment/QuestionTypes.tsx, and src/lib/api.ts.
• Blocked: targeted ESLint fails because eslint-plugin-unused-imports is missing from the repo install/config.

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