Polish Mast Sails home UX

[udaychandra]

Summary

• rename the SAIL environment panel to Sails and update Sail Home language
• redesign Sails rows, Sail Home header, command toolbar, empty/error states, and copy affordances
• remove outer rounding from side docks while preserving bottom dock rounding
• add the approved Mast/Sail square mark as the app icon source and regenerated PNG/ICO assets

Verification

• cargo fmt --check
• cargo check -p sing_project
• cargo check -p workspace
• cargo test -p sing_project
• git diff --check

Notes

• The app icon source uses the approved square dark-background SVG with #e85a30 as the brand accent.