Plot total subtests on WPT score comparison charts

[nicoburns]

Summary

• Add one gray dashed line showing the maximum total subtest count across browsers over time. At each date, take the maximum of each browser's latest available run.
• Plot the maximum and existing browser passing scores on the same percentage scale, using their shared fixed union denominator. No separate count axis; tooltips show both normalized percentage and raw total counts.
• Keep other history charts percentage-only.

Validation

• Formatting, cargo check --locked, JavaScript syntax, and git diff --check: passed.
• cargo clippy --all-targets --all-features --locked: passed with one pre-existing warning in src/downloads.rs.
• Screenshot inspected with real WPT data. Added tests remain removed; no automated tests or hover checks performed after removal.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/6d1fffc2ffdb4725a38ce82e39e623e5
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/6d1fffc2ffdb4725a38ce82e39e623e5?variant=devin-insiders
Requested by: @nicoburns

[staging-devin-ai-integration]

I'll fix CI failures and address comments from users with write access that start with 'Devin'.

• Disable automatic comment, CI, and merge conflict monitoring