fix(site/src/pages/AgentsPage): fix scroll-to-bottom pin starvation in agents chat

[mafredri]

scheduleBottomPin() cancelled any in-flight pin and restarted the double-RAF
chain on every ResizeObserver notification. When content height changes on
consecutive frames (streaming with SmoothText + markdown re-rendering), the
inner RAF that sets scrollTop is perpetually cancelled before it fires.

Proved via a frame-pipeline simulation that models the browser spec ordering
(RAF callbacks -> layout -> ResizeObserver callbacks): with the original
cancel-and-reschedule pattern, the pin never executes while content grows.
With the idempotent version, it fires every 2 frames.

Two fixes:

1. Make scheduleBottomPin() idempotent. If a pin is already in-flight,
   skip. The inner RAF reads scrollHeight at execution time so it always
   targets the latest bottom. User-interrupt paths (wheel, touch) still
   cancel via cancelPendingPins().

2. Add overscroll-behavior: contain to the scroll container. Prevents
   elastic overscroll from generating extra scroll events that could flip
   autoScrollRef to false.

The idle-after-chat-switch jumping is not addressed here. The initial mount
pin uses the same scheduleBottomPin() so fix #1 helps, but the root cause
of repeated jumps after navigation needs further investigation with Safari
instrumentation.

🤖 This PR was created with the help of Coder Agents, and will be reviewed by a human. 🏂🏻