Polish module copy and empty states

[mihape]

Summary

• make module headers and action labels clearer before the next release
• improve empty states so users know the next useful action
• keep the change limited to UI copy and low-risk markup text
• add the polish to the Unreleased changelog

Tests

• node --check src/modules/invoices/invoices.module.js
• node --check src/modules/fleet/fleet.module.js
• node --check src/modules/utilities/utilities.module.js
• node --check src/modules/attendance/attendance.module.js
• node --check src/modules/settings/settings.module.js
• git diff --check
• targeted render smoke test for updated module copy

Closes #37