fix(ssr): inject derived gradients and migrate component SCSS to CSS vars

The server-injected inline <style> was missing the three derived gradient
variables (--sky-gradient, --sunset-gradient, --mixed-gradient) that
components reference via var(). Without them, components fell back to
SCSS build-time defaults on the first paint, causing a visible flash
before ThemeService.apply() corrected the values.

Additionally, migrate three component SCSS files from hardcoded SCSS
color variables to CSS custom properties so the first paint uses the
server-injected runtime colors:
- login.component.scss: backgrounds, text colors, borders, focus rings
- events.component.scss: event date card background gradient
- about.component.scss: donation CTA background gradient

server.ts

@@ -129,10 +129,19 @@ function buildConfigScript(config: StationConfig): string {
   return `<script>window.__STATION_CONFIG=${json};</script>`;
 }
 
+/** Build a gradient CSS value from three color stops. */
+function gradient(from: string, mid: string, to: string): string {
+  return `linear-gradient(135deg, ${from} 0%, ${mid} 40%, ${to} 100%)`;
+}
+
 /**
  * Build inline CSS custom properties on `:root` so the first paint carries
  * the correct color scheme — eliminates the color flash that occurs when
  * ThemeService.apply() runs asynchronously after the API call.
+ *
+ * Includes both flat color properties AND derived gradient variables
+ * (--sky-gradient, --sunset-gradient, --mixed-gradient) so components
+ * using var() for backgrounds render correctly on the very first paint.
  */
 function buildThemeStyle(config: StationConfig): string {
   const props: string[] = [];
@@ -144,6 +153,15 @@ function buildThemeStyle(config: StationConfig): string {
     }
   }
 
+  // Derived gradients — mirror the same formulas ThemeService.apply() uses.
+  // These must be injected server-side; otherwise components referencing
+  // var(--sky-gradient) etc. fall back to their SCSS build-time defaults
+  // on first paint, causing a visible color flash.
+  const cp = (field: keyof StationConfig): string => (config as any)[field] || '';
+  props.push(`--sky-gradient: ${gradient(cp('color_primary_dark'), cp('color_primary'), cp('color_primary_light'))}`);
+  props.push(`--sunset-gradient: ${gradient(cp('color_accent_dark'), cp('color_accent'), cp('color_accent_light'))}`);
+  props.push(`--mixed-gradient: linear-gradient(135deg, ${cp('color_primary_dark')} 0%, ${cp('color_primary')} 50%, ${cp('color_accent')} 100%)`);
+
   if (props.length === 0) return '';
 
   const css = `:root{${props.join(';')}}`;