Add bundle size tracking to CI

[mikevitelli]

Context

next.config.ts includes withBundleAnalyzer but it only runs when ANALYZE=true (manual). There is no CI check for bundle size regressions.

Impact: Performance regressions can ship silently. No baseline to compare against.

Proposed approach

1. Add @next/bundle-analyzer output to CI artifacts
2. Use size-limit or bundlewatch to set thresholds
3. Post bundle size diff as PR comment (for preview PRs)
4. Warn if main JS bundle exceeds a defined threshold (e.g., 200KB gzipped)

Acceptance criteria

• Bundle size tracked on every CI run
• PR comments show size delta vs main branch
• Threshold violation fails CI or posts warning