fix: correct browsertime JSON paths for LCP, FCP, TTFB, navigation timings

- LCP/TTFB/TBT: read from statistics.googleWebVitals (flat median values)
- FCP: statistics.timings.paintTiming['first-contentful-paint']
- pageTimings: nested under statistics.timings.pageTimings, not at root
- Remove PerceptualSpeedIndex (not present in this sitespeed version)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

views/partials/timings.pug

@@ -8,7 +8,6 @@
 -   ['First Visual Change', job.first_visual_change],
 -   ['Last Visual Change', job.last_visual_change],
 -   ['Visual Complete 85%', job.visual_complete_85],
--   ['Perceptual Speed Index', job.perceptual_speed_index],
 -   ['Front End Time', job.front_end_time],
 -   ['Back End Time', job.back_end_time],
 -   ['Max Potential FID', job.max_potential_fid],