Capture phase for Autocapture listeners

• Registers autocapture click/change/submit listeners in the capture phase, so events are reliably observed even when downstream handlers call stopPropagation().
• Fixes a session recording bug where record_min_ms could be bypassed after a page navigation or an idle period, causing short recordings to be uploaded. Elapsed time is now measured from the first rrweb event timestamp rather than wall-clock time, so the minimum-duration check stays accurate across navigations.
• Fixes a malformed first-time-events request URL that contained a double slash (//) before the flag ID. The URL is now built cleanly regardless of whether the base ends in /.