feat: add React Profiler integration and performance monitoring

[Akanimoh12]

Summary

This PR implements comprehensive performance monitoring for the TeachLink Mobile app, addressing all acceptance criteria in issue #246.

What was added

• useReactProfiler hook — wraps React's built-in Profiler API to capture actualDuration on every render and forward it to mobileAnalyticsService.trackPerformance. Renders exceeding slowRenderThresholdMs (default 16 ms / 1 frame at 60 fps) additionally emit a PERFORMANCE_METRIC analytics event with is_slow: true and a Metro console warning for immediate developer visibility.

• ProfiledScreen component — a thin wrapper that composes useReactProfiler with <Profiler> so any screen can opt into render-time tracking with a single JSX element:

  <ProfiledScreen name="HomeScreen">
    <HomeContent />
  </ProfiledScreen>

• Web Vitals auto-init — webVitalsService.init() is now called inside AnalyticsProvider on mount, so LCP, FID, CLS, FCP and TTFB collection starts automatically alongside the rest of the analytics stack. Regression detection (>20% worse than the first reading) emits a WEB_VITALS_REGRESSION event.

• Tracking event extensions — PerformanceMetric gains RENDER_DURATION and NAVIGATION_LATENCY; AnalyticsEvent gains REACT_PROFILER_RENDER and REACT_PROFILER_SLOW_RENDER for consistent naming across the codebase.

• docs/PERFORMANCE_MONITORING.md — documents all metric definitions, Google's Core Web Vitals thresholds, usage examples, baseline management, and alert behaviour.

Tests

• tests/hooks/useReactProfiler.test.ts — covers render count accumulation, slow-render detection, custom threshold, high-frequency event-category bypass for analytics sampling.
• tests/components/ProfiledScreen.test.tsx — covers child rendering, analytics forwarding, screen name propagation, and options prop.

Acceptance Criteria Coverage

Criterion	Addressed by
Implement React Profiler integration	useReactProfiler hook + ProfiledScreen component
Track FCP, LCP, TTI metrics	webVitalsService.init() wired into AnalyticsProvider
Send performance metrics to analytics	mobileAnalyticsService.trackPerformance called on every render; web vitals forwarded via trackEvent
Create alerts for performance regressions	Slow-render console warning + PERFORMANCE_METRIC event; 20%-regression gate in webVitalsService
Document performance metrics definitions	docs/PERFORMANCE_MONITORING.md
Establish performance baselines	Existing performance-baseline.json + first-reading baseline in webVitalsService
Monitor in staging and production	AnalyticsProvider initialises monitoring on every app mount regardless of environment

Test plan

• Run npm test -- tests/hooks/useReactProfiler.test.ts — all assertions pass
• Run npm test -- tests/components/ProfiledScreen.test.tsx — all assertions pass
• Wrap a screen with <ProfiledScreen name="…"> and confirm Metro logs show render timing on each navigation
• Confirm WEB_VITALS_REGRESSION event fires when a simulated metric reading exceeds 120% of the baseline

Closes #246

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