feat: expand telemetry — Sentry spans, GA4 events, Sentry init fix

[thostetler]

Next.js 16 Turbopack bypasses the Sentry webpack plugin, so sentry.client.config.ts was never loaded in dev. This adds a runtime guard to require it manually when Turbopack is active.

Instruments key user flows with Sentry performance spans and GA4 behavioral events for production latency and engagement tracking.

Sentry spans:

• search.submit.total, search.pagination.total — query lifecycle
• search.results.render, search.facets.render — paint timing after docs arrive
• export.generate.total — full export flow
• abstract.load.total, abstract.citations.load, abstract.references.load — SSR fetches
• auth.login.total, auth.register.total, auth.session.validate — auth flows
• orcid.oauth.total, orcid.claim.total, orcid.sync.total — ORCID flows
• ux.full_text_time — time from search results to full-text link click

GA4 events:

• result_click — user clicks an abstract link
• full_text_click — user opens a full-text resource (mark as conversion in GA4)
• abstract_tab_view — user switches abstract tabs
• search_no_results — query returns zero results

Note: full_text_click must be manually marked as a key event in GA4 Admin > Events.

[codecov]

Codecov Report

❌ Patch coverage is 61.84971% with 66 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.6%. Comparing base (da070fc) to head (afe019d).

Files with missing lines	Patch %	Lines
src/lib/performance.ts	54.5%	31 Missing ⚠️
src/api/orcid/orcid.ts	0.0%	11 Missing ⚠️
...mponents/ResultList/Item/ItemResourceDropdowns.tsx	47.7%	11 Missing ⚠️
src/lib/useExportSpan.ts	72.3%	10 Missing ⚠️
src/lib/serverside/absCanonicalization.ts	86.7%	2 Missing ⚠️
...components/CitationExporter/useCitationExporter.ts	94.5%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #859     +/-   ##
========================================
- Coverage    62.7%   62.6%   -0.0%     
========================================
  Files         323     324      +1     
  Lines       38011   38143    +132     
  Branches     1723    1728      +5     
========================================
+ Hits        23803   23876     +73     
- Misses      14168   14226     +58     
- Partials       40      41      +1     

Files with missing lines	Coverage Δ
src/lib/serverside/bootstrap.ts	76.6% <100.0%> (+0.6%)	⬆️
...components/CitationExporter/useCitationExporter.ts	91.0% <94.5%> (+0.4%)	⬆️
src/lib/serverside/absCanonicalization.ts	64.3% <86.7%> (+0.7%)	⬆️
src/lib/useExportSpan.ts	72.3% <72.3%> (ø)
src/api/orcid/orcid.ts	26.6% <0.0%> (-0.2%)	⬇️
...mponents/ResultList/Item/ItemResourceDropdowns.tsx	77.1% <47.7%> (-1.7%)	⬇️
src/lib/performance.ts	78.3% <54.5%> (-10.6%)	⬇️

... and 6 files with indirect coverage changes

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