Refine chat activity loader layout

[Afnanksalal]

What changed

• Force chat_activity into an explicit vertical child layout so it no longer inherits horizontal chat-bubble flow.
• Bound the content width and render two quiet skeleton/shimmer lines instead of one stretched rail.
• Keep the public function signature unchanged for Tokito consumers.

References

• Material progress indicators: loading should be communicated without blocking context.
• Carbon loading guidance: avoid noisy stacks of loaders; skeleton states fit progressive content.
• NN/g skeleton screens: layout-like placeholders reduce perceived waiting.

Verification

• cargo fmt --all
• cargo clippy --all-targets -- -D warnings
• RUSTDOCFLAGS='-D warnings' cargo doc --no-deps