Keep composer usable with file drops

[oshtz]

Summary

• keep composer controls usable when a wide inspector leaves a narrow chat column
• accept OS file drops through the existing composer attachment flow
• prepare v0.2.28 as a summary-only release with no update cards

Verification

• git diff --check
• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo clippy --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets -- -D warnings
• full Tauri library tests with the Windows test manifest flags (68 passed)
• pnpm -C apps/desktop verify
• pnpm -C apps/site build