feat: add CSS custom properties design system

[wowitsjack]

Full source available at FredTV-Next, my personal fork with all features integrated.

Summary

• Introduce centralized CSS custom properties (design tokens) in styles.css
• Replace all hardcoded color values across 6 component CSS files with variable references
• Tokens cover: surfaces, borders, text, accent, semantic colors, radius, transitions, shadows
• Add Material menu, modal animation, and toastr dark theme overrides
• Visual appearance is unchanged (variables resolve to equivalent or improved dark theme values)

Files changed:

• src/styles.css - :root custom properties + global overrides
• src/app/channel-tile/channel-tile.component.css - color variables + favorite star pop animation
• src/app/download-manager/download-manager.component.css
• src/app/epg-modal/epg-modal.component.css - + arrow hover effect
• src/app/epg-modal/epg-modal-item/epg-modal-item.component.css
• src/app/settings/source-tile/source-tile.component.css
• src/app/setup/confirm-modal/confirm-modal.component.css

Test plan

• Verify dark theme appearance is consistent across all views
• Check channel tiles, EPG modals, download manager, settings source tiles, confirm modals
• Verify Material context menus have rounded corners and dark background
• Check toastr notifications match dark theme

[CLAassistant]

All committers have signed the CLA.

[wowitsjack]

Yep, signed

[wowitsjack]

Superseded by #396 + #397 which contribute the full FredTV-Next codebase as a proper 2-part stack.