feat: add PostHog analytics integration

[posthog]

Summary

This PR ships the PostHog setup wizard's latest pass over the existing posthog-js integration (v1.288.0 → v1.410.10 within the same ^1.288.0 semver range) in vite-project:

• src/main.tsx — guards the existing PostHogProvider wrap: renders the app without it (skipping the crash-when-missing-env-var mentioned below) instead of unconditionally passing possibly-empty apiKey/api_host, and throws a loud dev-only error if VITE_PUBLIC_POSTHOG_KEY/VITE_PUBLIC_POSTHOG_HOST are unset.
• src/features/auth/AuthContext.tsx — adds an account-switch guard: posthog.reset() now fires when a different Firebase uid logs in, not just on logout, so identity doesn't get merged across accounts.
• Event captures added at successful-action boundaries, all non-PII product properties:
  • src/components/elevationfinder/GpxUploader.tsx — gpx_route_uploaded
  • src/features/elevation/components/RouteDashboard.tsx — route_analysis_settings_updated
  • src/features/fuel/components/AIPersonalization.tsx — fuel_ai_advice_requested
  • src/features/fuel/components/FuelPlannerV2.tsx — fuel_plan_calculated
  • src/features/fuel/hooks/useFuelPlanPersistence.ts — fuel_plan_saved
  • src/features/goals/components/OnboardingFlow.tsx — onboarding_completed
  • src/features/pace-calculator/components/PaceCalculatorV2.tsx — pace_calculated
  • src/features/pace-calculator/hooks/usePacePlanPersistence.ts — pace_plan_saved
  • src/features/plan/components/PlanInputForm.tsx — training_plan_generated
  • src/features/plan/hooks/useSavePlan.ts — training_plan_saved
• .env.example — documents VITE_PUBLIC_POSTHOG_KEY / VITE_PUBLIC_POSTHOG_HOST.
• package-lock.json — refreshes posthog-js and its transitive deps within the existing package.json version range.

Insights and dashboards created

Type	Name	Link
Dashboard	Analytics basics (wizard)	https://us.posthog.com/project/524328/dashboard/1951948
Insight	Runner activity (wizard)	https://us.posthog.com/project/524328/insights/DjdwINiL
Insight	Pace save funnel (wizard)	https://us.posthog.com/project/524328/insights/pdHOBIqL
Insight	Fuel plan save funnel (wizard)	https://us.posthog.com/project/524328/insights/GhVdZ5Se
Insight	Training plan save funnel (wizard)	https://us.posthog.com/project/524328/insights/5ah2hnB8
Insight	Route and AI engagement (wizard)	https://us.posthog.com/project/524328/insights/zu6vIQJu

How to verify

1. Deploy this branch (or run it locally with the env vars below already set in vite-project/.env).
2. Log in/out with a Google account and confirm no console errors from the main.tsx env-var guard.
3. Exercise a few flows — calculate a pace, generate a fuel plan, upload a GPX route, finish onboarding.
4. In PostHog, open Activity and confirm the corresponding events (pace_calculated, fuel_plan_calculated, gpx_route_uploaded, onboarding_completed, etc.) arrive within a minute.

Environment variables

TrainPace deploys to Vercel (per vercel.json / CLAUDE.md), which keeps environment variables in its dashboard rather than in a file committed to this repo, so I could not set these up automatically. VITE_PUBLIC_POSTHOG_KEY and VITE_PUBLIC_POSTHOG_HOST were already in use before this PR (see src/main.tsx on main), so production should already have them configured — this is a reminder to confirm, not a first-time setup:

Name	Value
VITE_PUBLIC_POSTHOG_KEY	phc_vM6GXKZAALbMzPuM8vfRsD2zaayjC7VL48kqLMWFZJNQ
VITE_PUBLIC_POSTHOG_HOST	https://us.i.posthog.com

To confirm or set them:

1. Open https://vercel.com/dashboard and select the TrainPace project.
2. Go to Settings → Environment Variables.
3. Confirm both rows above exist for the Production environment (add them if missing), then Save.
4. If you add/change anything, redeploy (Deployments tab → "..." on the latest deployment → Redeploy) — env var changes only take effect on the next deployment.

Setting environment variables via a local CLI

echo "phc_vM6GXKZAALbMzPuM8vfRsD2zaayjC7VL48kqLMWFZJNQ" | npx vercel env add VITE_PUBLIC_POSTHOG_KEY production
echo "https://us.i.posthog.com" | npx vercel env add VITE_PUBLIC_POSTHOG_HOST production

---

Created with PostHog Code

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[codacy-production]

Not up to standards ⛔

🔴 Issues 19 high

Alerts:
⚠ 19 issues (≤ 0 issues of at least minor severity)

Results:
19 new issues

Category	Results
ErrorProne	19 high

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🟢 Metrics 0 complexity · 0 duplication

Metric	Results
Complexity	0
Duplication	0

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[posthog]

Codacy Static Code Analysis shows action_required (19 new issues, 0 max.) — all 19 are no-unsafe-call / no-unsafe-member-access warnings on the new posthog.capture(...) lines this PR adds, e.g. AIPersonalization.tsx#L73.

This looks like a Codacy-isolated-analysis artifact rather than a real type error: npm run build (tsc -b) and npm run lint (eslint .) both pass cleanly on these exact lines in this repo's own toolchain, and it's not a required check here — recent merged PRs (#77, #78) show the same action_required conclusion on unrelated changes. Leaving this as-is rather than modifying the wizard-authored instrumentation code to work around a third-party tool's type resolution.