fix(web-ui): stop animating programmatic transcript scrolls

[time-attack]

Problem

When a run ended with a pending approval, the web-ui transcript visibly scrolled from the top of the conversation all the way to the bottom as an animation.

The transcript scroller (.chat-scroll) had scroll-behavior: smooth. That property only affects programmatic scrolls (assigning scrollTop), never wheel or trackpad input. Every redraw that re-sticks the transcript to the bottom therefore animated, and when the scroller had been rebuilt at the top the animation covered the whole conversation.

Change

• Remove scroll-behavior: smooth from .chat-scroll.
• Delete the three call sites that flipped style.scrollBehavior to auto and back to dodge the animation (forced scroll-to-bottom, and both pagination anchor adjustments). They are now plain scrollTop writes.
• The layout test now asserts no smooth scrolling in the transcript CSS and no scrollBehavior juggling in chat.ts, and still asserts both pagination anchors exist.

Net: 5 insertions, 20 deletions.

Demo

Behavior-only change: the after state renders pixel-identical to before, the difference is that scroll-to-bottom is now instant instead of animated. No screenshot can show it, so none attached. To see it: open a long conversation, trigger a run that ends in an approval, and observe the transcript snaps to the pending approval instead of animating from the top.

Verification

• plugins/web-ui: layout-thrash, tab-switch-scroll-snap, pane-approval-overflow tests pass.
• plugins/web-ui typecheck passes; eslint clean on changed files.

Not in this PR

The screenshot that prompted this also asked for an "allow always" option on security-screen quarantine releases. Those are deliberately once-only in the orchestrator (server-side guard plus tests). The existing knob is the security-posture admin resource (dangerous disables content screening). Left unchanged here.

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[time-attack]

Independent review pass (code-review, medium, eight angles) on the first commit found no correctness issues. Two cleanups it raised are applied in the follow-up commit:

• The two pagination anchor-restore blocks were verbatim duplicates once the smooth-scroll workaround was gone. They now share one holdScrollAnchor helper.
• The layout test no longer pins the duplication at exactly two copies. It asserts both loaders route through the helper.

Confirmed root cause from the altitude angle: mountContinuable and mountReadOnly build a fresh host at scrollTop 0 and snap to the bottom a frame later, which scroll-behavior: smooth turned into a full-transcript animation. Removing the CSS rule fixes every current and future programmatic scroll on that element in one place.

Left as pre-existing and out of scope: scrollTranscript has no pending-rAF coalescing guard, and the two mount paths duplicate the build-then-snap shape.