fix(sentry): correct timing calculations in performance spans

[thostetler]

performance.now() returns milliseconds relative to page load, not
epoch timestamps. The duration calculation was subtracting epoch ms
from relative ms (producing garbage), and span.end() was receiving
relative seconds instead of epoch seconds (causing Sentry to drop
the spans silently).

• Fixed setMeasurement to use performance.now() directly (already relative to navigation start)
• Fixed span.end() to convert relative time to epoch seconds

[Copilot]

Pull request overview

Fixes incorrect Sentry performance span timing by aligning relative performance.now() values with epoch-based timestamps, preventing invalid durations and dropped spans.

Changes:

• Update timing.results.shown measurement to use performance.now() directly (ms since navigation start).
• Convert span end time from relative seconds to epoch seconds by offsetting with performance.timeOrigin / performance.timing.navigationStart.

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