Add browser performance report to the merge request reports tab

What does this MR do and why?

Adds BrowserPerformanceProvider, BrowserPerformanceNavItem, BrowserPerformancePage, and BrowserPerformanceContent, following the code quality report pattern, plus a View report button on the browser performance widget. The comparison, formatting, and summary logic moves into a shared utils.js used by both the widget and the report. The shared ReportSection component now renders summary.subtitle, which the browser performance summary needs for its counts line.

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