vehicles: don't see your car? button

[sshane]

PostHog, 7 day clean window, 17,923 /vehicles sessions.

• 96.6% of pageviews scroll past the "Don't see your car?" card, which sits at ~12% depth. Most people are asked before they could have found their car missing.
• The card converts at 1.02% against ~97% reach. Timing is the constraint, not reach.
• 41.2% of sessions open zero car rows, and produced 1 signup between them.
• Checkout rate falls with depth: 3.40% shallow, 1.14% at the last brand. The people who work the whole list convert worst.
• 96.3% of card submits fill in the optional car field, so signups here carry a make and model.
• The zero-row cohort's median dwell is 20s vs 50s for people who open rows, and 15.8% of pageviews scroll back up by more than 40%.

Adds a floating "don't see your car?" button on /vehicles. It appears after 15s in the car list, 10s if they jumped there from the brand grid, or immediately after scrolling back up two screens. It links to the existing card.

Caveat: Buy now on this page is a nav link and only 14.24% of its clicks reach Checkout, so every rate above uses Checkout clicks.

[github-actions]

deployed preview: https://comma-web--pr388-7xhc2iol.web.app

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