Report: extend the trend charts (flaky count, link to a past run) #165

Follow-up to #160. Ideas: a flaky-test count line, and clicking a point to open that run's report if it was kept. Pick one and propose it here first.

Done when: the chosen addition renders from metrics history and is hidden when the data isn't there.

Relevant code: src/main/resources/report-template.html (markup, CSS, Chart.js) and src/main/java/com/seleniumboot/reporting/HtmlReportGenerator.java (fills the {{...}} placeholders). Build with mvn clean verify, then open target/selenium-boot-report.html. Comment here before starting so we don't double up.