fix: stabilize Stay overlays and guard borders

[avatarneil]

Summary

Stay overlays now respond faster and cover the right display surfaces while guarding. The guard handle avoids redundant native resize calls, the locked overlay can scroll on constrained windows instead of clipping, and the blue guard border is created per display with each monitor's own scale-aware coordinates.

The launch intro animation is intentionally unchanged.

Performance And Behavior

Area	Before	After
Guard handle reveal	repeated hover/focus events could invoke native resize repeatedly	repeated events are idempotent and only real state changes call native resize
Guard handle collapse	180 ms hover-out delay	60 ms hover-out delay
Shell motion	transitions included layout/paint-heavy properties	transitions stay on compositor-friendly properties
Locked overlay	constrained overlays clipped content	constrained overlays scroll inside the panel
Guard border	one border window followed the main Stay window's monitor	one border window is positioned for each available display
Mixed-scale displays	secondary borders could be placed through the primary display scale space	border placement uses each target display's scale factor

Validation

• bun run test
• bun run typecheck
• cargo test
• bun run frontend:build
• git diff --check

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