Remove chat activity shimmer bars

[Afnanksalal]

What changed

• Remove the two shimmer/skeleton bars from chat_activity.
• Keep the compact Thinking/status row, breathing dots, and detail copy.
• Update README wording so the component docs match the simpler state.

Verification

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