Fix README workflow graphic and restore original icons

[daredoole]

Summary

• replace the broken README mock screenshot with a clear architecture diagram
• show the tray and desktop controller as two control surfaces for the mwb-client service
• shorten and wrap copy so nothing clips or overlaps at 1280×720 or typical README width
• improve the image’s accessible title, description, and README alt text
• revert the unrelated icon/mascot replacement from PR Replace mascot with clear device-flow icon #20 and restore the original assets

Validation

• visually inspected at 1280×720 and 800×450
• SVG passes xmllint
• README image reference and accessible alt text verified
• git diff --check passed
• public repository privacy audit passed
• original icons restored locally; KDE cache refreshed; tray restarted and active