Display timestamps in browser local time

[bradleymackey]

Summary

• render visible instants through a shared semantic local-time component with canonical UTC metadata and an explicit UTC fallback
• localize timestamps with the browser's locale and timezone on initial load and after HTMX swaps
• cover issue, project, comment, changelog, sprint-history, context, token, passkey, and GitHub integration timestamps without changing API or storage values

Validation

• go test -count=1 ./internal/server
• node --check internal/server/static/app.js
• exercised Intl.DateTimeFormat across the Europe/London DST transition and a local day boundary
• DATABASE_URL=postgres://track:track@localhost:5436/track?sslmode=disable go test -count=1 -coverprofile=/tmp/track38-server-cover.out ./internal/server (67.0%)