feat: switch to light green + deep purple theme, fix input colors

- Primary: emerald green (#059669), accent: deep purple (#7c3aed)
- Light backgrounds: #f0fdf4 page, #ffffff cards
- Login/Register: light gradient bg, white card, Element Plus inputs
- NotFound: matching light theme
- All pages follow via CSS variables

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/views/Layout.vue

@@ -214,8 +214,8 @@ function goHome() {
 }
 
 .nav-item--active {
-  background: rgba(99, 102, 241, 0.15);
-  color: var(--gg-primary-light);
+  background: rgba(5, 150, 105, 0.1);
+  color: var(--gg-primary);
 }
 
 .nav-icon {
@@ -295,7 +295,7 @@ function goHome() {
 }
 
 .group-item--active {
-  background: rgba(99, 102, 241, 0.15);
+  background: rgba(5, 150, 105, 0.1);
   border-left: 3px solid var(--gg-primary);
 }