Add Core Web Vitals distribution histogram to tech report

[alonkochba]

Summary

• Adds a collapsible histogram chart to the CWV section of the tech report drilldown page
• Shows how origins are distributed across performance buckets for LCP, CLS, INP, FCP, and TTFB
• Bars are color-coded green/orange/red by good/needs-improvement/poor thresholds
• Dashed plotlines mark threshold boundaries
• Tail buckets are aggregated into an overflow "X+" bar so all origins are visualized
• Supports light and dark mode
• Collapsed by default via native <details>/<summary> element
• Fetches data from /v1/cwv-distribution API (Add /v1/cwv-distribution endpoint tech-report-apis#105), falls back to bundled sample data until the endpoint is deployed

Depends on

• Add /v1/cwv-distribution endpoint tech-report-apis#105 — adds the /v1/cwv-distribution BigQuery endpoint

Pending

• Remove bundled cwvDistributionData.json fallback once the API endpoint is live

Test plan

• Navigate to /reports/techreport/tech?tech=Wix
• Expand "Core Web Vitals distribution" in the CWV section
• Verify histogram renders with colored bars and threshold lines
• Switch between LCP, CLS, INP, FCP, TTFB metrics
• Toggle desktop/mobile client
• Switch to dark mode and verify colors/contrast
• Verify the overflow "X+" bucket appears at the tail
• Verify existing timeseries charts still render correctly
• Verify histogram does not appear on the comparison page

Closes #1147