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

src/app/about/about.component.scss

@@ -213,7 +213,7 @@
 .about-cta {
   text-align: center;
   padding: $spacing-2xl $spacing-lg;
-  background: $mixed-gradient;
+  background: var(--mixed-gradient);
   border-radius: $radius-xl;
   color: var(--color-white);
 

src/app/events/events.component.scss

@@ -34,7 +34,7 @@
 
   .event-date {
     flex-shrink: 0;
-    background: $mixed-gradient;
+    background: var(--mixed-gradient);
     color: var(--color-white);
     display: flex;
     flex-direction: column;

src/app/login/login.component.scss

@@ -4,7 +4,7 @@
 .login-section {
   @include flex-center;
   min-height: calc(100vh - 80px);
-  background: $sky-gradient;
+  background: var(--sky-gradient);
   padding: $spacing-xl;
 }
 
@@ -21,13 +21,13 @@
 
   h1 {
     font-family: $font-heading;
-    color: $primary-blue;
+    color: var(--color-primary);
     margin: $spacing-sm 0 $spacing-xs;
     font-size: 1.75rem;
   }
 
   p {
-    color: $neutral-medium;
+    color: var(--color-medium);
     font-size: 0.9rem;
   }
 }
@@ -49,14 +49,14 @@
     display: block;
     font-size: 0.875rem;
     font-weight: 600;
-    color: $neutral-dark;
+    color: var(--color-dark);
     margin-bottom: $spacing-xs;
   }
 
   input {
     width: 100%;
     padding: $spacing-sm $spacing-md;
-    border: 1px solid $neutral-light;
+    border: 1px solid var(--color-light);
     border-radius: $radius-md;
     font-size: 1rem;
     transition: border-color $transition-fast;
@@ -64,7 +64,7 @@
 
     &:focus {
       outline: none;
-      border-color: $primary-blue;
+      border-color: var(--color-primary);
       box-shadow: 0 0 0 3px rgba($primary-blue, 0.15);
     }
   }
@@ -72,7 +72,7 @@
 
 .login-error {
   background: rgba($danger-red, 0.1);
-  color: $danger-red;
+  color: var(--color-danger);
   padding: $spacing-sm $spacing-md;
   border-radius: $radius-md;
   font-size: 0.875rem;
@@ -81,7 +81,7 @@
 }
 
 .btn-login {
-  @include button-style($primary-blue, $neutral-white, $primary-blue-light);
+  @include button-style(var(--color-primary), var(--color-white), var(--color-primary-light));
   width: 100%;
   padding: $spacing-sm $spacing-lg;
   margin-top: $spacing-sm;