perf(site): cut rendering cost of long-lived UI animations

[tracyjohnsonux]

Long-lived infinite animations in the Agents UI keep Chrome rendering at display refresh rate even when the page is otherwise idle. This is a broader pass over the same problem as #28182, replacing each hot animation with a technique that is cheap by construction rather than stepping the existing smooth arc, so nothing reads as choppy.

• Spinners: the shared segmented Spinner now uses a single steps(8) rotation over a static opacity gradient (10 updates/s, one animation) instead of eight per-leaf opacity animations running at refresh rate. Long-lived Agents spinners (sidebar running status, running tool calls, subagents, build logs, plan/question submits) switch from smooth lucide animate-spin arcs to this Spinner; discrete stepping on 8-leaf geometry is the classic activity-indicator look, so nothing appears janky. Global animate-spin is untouched for transient uses.
• Text shimmer ("Thinking" and active tool labels): replaced the framer-motion backgroundPosition loop (JS work every frame, one loop per active tool) with a pure CSS keyframe that sweeps for 60% of a 3.5s cycle and holds for the rest, so the browser runs no per-frame JS and can skip repaints between sweeps. Reduced motion now parks the highlight off the text.
• Task prompt caret sweep: animated left (layout each frame) with four stacked large-blur box-shadows; now a compositor-friendly translateX sweep (via an inline-size container and 100cqw) with a gradient trail instead of blurred shadows.
• LinearProgress indeterminate bars: ported the MUI-derived left/right keyframes to translateX + scaleX so they composite instead of triggering layout every frame.

Story assertions that matched the .animate-spin class now query the spinner's accessible title. The pre-existing MCP Tool Completed story failure reproduces on main and is unrelated.

Refs #28182

This pull request was generated by Coder Agents on behalf of @tracyjohnsonux.

Audit notes: remaining animation inventory

• loading keyframe (opacity pulse) is compositor-friendly and left as is.
• spin-once (GitPanel refresh) and dialog animate-in/out are one-shot and left as is.
• SmoothText streaming reveal and scroller rAF loops are bounded and stop when idle.
• zip-right animates left/width but is one-shot (1s, non-infinite), so it was left alone.