feat: replace plausible analytics with opentelemetry browser instrumentation

[visoedhwa]

Close #19

[arolos]

• Events lost on tab close — BatchLogRecordProcessor never flushed on pagehide/visibilitychange.
• Duplicate click events — nav_click + generic a[href] handler both fire content_section for the same hash link.
• Scroll re-emits content_section after a click already emitted it (lastTrackedSection only updated in scroll path).
• Inconsistent page_type — view_toggle computes its own variant that doesn't strip .html or normalize index.
• Duplicated getPageType in js/analytics/init.js and js/page-setup.js.
• Heartbeat interval never cleared, no session_end ever emitted.
• Hostname-based isProduction treats every non-localhost host (staging, PR previews) as prod.
• Wrong DNS resolver for K8s — resolver 127.0.0.11 is Docker's embedded DNS, not CoreDNS.
• Hard-coded collector address opentelemetry-collector.observability.svc:4318 not templated via Helm.
• AGENTS.md says Plausible, code uses OpenTelemetry.