fix: keep workspace toast stacks scrollable

[oz-for-oss]

Closes #9733

Summary

• Constrain the workspace global toast overlay by the remaining window height instead of only repositioning oversized stacks.
• Render DismissibleToastStack through a clipped vertical scrollable with an overlay scrollbar when toasts are present.
• Reset the toast stack scroll position when a new toast is added so the newest toast remains visible.

Validation

• cargo fmt --check
• cargo check -p warp --lib
• CARGO_BUILD_JOBS=1 LIBCLANG_PATH=/usr/lib/llvm-18/lib cargo clippy -p warp --lib --all-features -- -D warnings

Notes

• Full cargo clippy --workspace --all-targets --all-features --tests -- -D warnings initially failed because libclang was missing in the sandbox. After installing libclang and rerunning with LIBCLANG_PATH=/usr/lib/llvm-18/lib, the full workspace/test clippy invocation was killed by SIGKILL during warp lib test compilation, likely due to sandbox memory limits. The targeted app-library clippy pass above succeeded.
• No approved spec context was available; this implements the smallest general toast-container fix for the reported overflow.