feat: refactor components to use slots, implement CSS-controlled SVG graphics, and enhance theming system

- Refactored InitialSection.astro to use named slot (extraContent) instead of prop for flexible HTML/JSX content injection from parent components
- Updated index.astro to utilize extraContent slot in InitialSection component
- Replaced decorative background div in Layout.astro with inline SVG element, enabling dynamic color control via CSS variable (--naliia-n-bg-color)
- Added .bg-svg class and --bg-svg-color variable in global.css for customizable SVG decorative background
- Converted navbar logo from img to inline SVG with CSS-controlled gradient using linearGradient definition
- Implemented gradient variables (--navbar-n-naliia-start, --navbar-n-naliia-end) for theme-aware logo coloring
- Configured vertical gradient (bottom-to-top) with pink color at bottom for navbar logo
- Added data-theme attribute support in Layout.astro for light/dark theme switching
- Extended global.css with comprehensive color palette (pink, yellow, mistyRose variants) and semantic tokens
- Refactored all color references to use CSS custom properties for consistent theming
- Updated Footer.astro logo from img to inline SVG with gradient support matching navbar implementation
- Replaced hardcoded color values in navbar.css and footer.css with semantic token variables
- Added [data-theme='dark'] selector in global.css to support future dark mode implementation
- Created reusable CSS variables for borders, backgrounds, and text colors across components
- Improved pricing card layout with enhanced visual hierarchy and gradient-based pricing tiers
- Restructured index.astro pricing section with improved feature presentation and layout

src/components/InitialSection.astro

@@ -41,6 +41,8 @@ const { imgSrc, imgAlt, logoSrc, logoAlt, mainTitle, subtitle, buttonText } =
                 )
             }
         </div>
+
+        <slot name="extraContent" />
     </div>
 </section>