fix: isolate typing indicator animation with geometryGroup to prevent position jumping

[devin-ai-integration]

Summary

Adds .geometryGroup() to TypingIndicatorView to isolate its repeatForever scale animation from parent geometry changes.

Problem: The three-dot typing indicator dots bounce erratically in position instead of gently pulsing in scale. This surfaced after PR #24530 changed the outer MessageListView frame from .frame(maxWidth:).frame(maxWidth: .infinity) (stable full-width FlexFrame) to .frame(width: computed) (FrameLayout whose width shifts when containerWidth updates from 0→actual).

Hypothesis: The .animation(.easeInOut.repeatForever, value: animate) on each Circle creates a persistent animation context. When parent geometry changes reach the view (LazyVStack repositioning, outer frame resize, ChatView's .animation(VAnimation.fast, value: btwResponse) on the mainContentStack), the persistent context captures those unrelated geometry changes, causing position bouncing.

Fix: .geometryGroup() acts as a barrier — parent geometry changes are resolved to final values before being passed to children, preventing the repeatForever animation from interacting with external layout transitions.

⚠️ This fix is based on code analysis only — it could not be tested locally (native macOS/iOS app). The hypothesis is plausible but not empirically confirmed.

Review & Testing Checklist for Human

• Dots stop jumping: Open a conversation in thinking state. Verify the three dots pulse gently in scale within their bubble background — no erratic vertical/horizontal bouncing. Test on both fresh launch and conversation switch.
• Animation still looks correct: The staggered easeInOut wave pattern (0.18s offset per dot) should be preserved. No stuttering, no delayed animation start, no frozen dots.
• No layout regression from .geometryGroup(): The typing indicator bubble should render at the correct position and size. Check that the bubble background (rounded rectangle) still wraps the dots tightly with correct padding. .geometryGroup() can subtly affect geometry propagation timing.
• iOS build and behavior: This file is in clients/shared/. Verify it compiles and the indicator behaves correctly on iOS as well (if applicable).

Notes

• If .geometryGroup() doesn't fix the jumping, the next approach would be replacing the .animation(_:value:) + repeatForever pattern with PhaseAnimator (macOS 14+/iOS 17+), which is inherently scoped and cannot leak animation context.
• The indicator code in MessageListContentView (.frame(width: effectiveBubbleMaxWidth) on the indicator rows) has not changed — this fix targets the animation isolation, not the container sizing.

Link to Devin session: https://app.devin.ai/sessions/ef66aadc67724413b3b5ede76cf20ca9
Requested by: @ashleeradka

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.