Release 1.7.9 CSS modularization

[Sportinger]

Summary

• Split the App.css monolith into owner-focused CSS files for timeline, panels, preview, export, common dialogs, and shared app foundations.
• Added cascade-layered global styles and removed src/App.css/src/index.css.
• Bumped APP_VERSION to 1.7.9 and added the changelog entry.

Closes #131.

Verification

• npm run build
• npm run lint (passes with existing EditableDraggableNumber warnings)
• npm run test
• Manual UI smoke check on the dev server