fix(styles): resolve design system audit violations

- Add --color-text-on-accent token to tokens.css
- Migrate all hardcoded color:#fff/#ffffff to var(--color-text-on-accent) across all module stylesheets
- Fix toggle thumb background (#fff -> var(--color-surface)) for dark mode
- Migrate hardcoded transition durations to token vars (--transition-fast/base/slow)

public/styles/dashboard.css

@@ -94,7 +94,7 @@
   background: linear-gradient(135deg, var(--greeting-from), var(--greeting-to));
   border-radius: var(--radius-md);
   padding: var(--space-4) var(--space-5);
-  color: #ffffff;
+  color: var(--color-text-on-accent);
   grid-column: 1 / -1;
 }
 
@@ -132,7 +132,7 @@
   align-items: center;
   gap: var(--space-1);
   background: var(--color-glass);
-  color: #ffffff;
+  color: var(--color-text-on-accent);
   font-size: var(--text-xs);
   font-weight: var(--font-weight-medium);
   padding: 2px var(--space-2);
@@ -199,7 +199,7 @@
   align-items: center;
   justify-content: center;
   background-color: var(--color-accent);
-  color: #ffffff;
+  color: var(--color-text-on-accent);
   font-size: var(--text-xs);
   font-weight: var(--font-weight-bold);
   min-width: 16px;
@@ -313,7 +313,7 @@
   justify-content: center;
   font-size: var(--text-xs);
   font-weight: var(--font-weight-bold);
-  color: #ffffff;
+  color: var(--color-text-on-accent);
   flex-shrink: 0;
   margin-top: 1px;
 }
@@ -537,7 +537,7 @@
   height: 100%;
   background-color: var(--color-success);
   border-radius: var(--radius-full);
-  transition: width 0.3s ease;
+  transition: width var(--transition-base);
 }
 
 .shopping-widget-list__items {
@@ -642,7 +642,7 @@
 .weather-widget {
   position: relative;
   background: linear-gradient(135deg, var(--color-accent) 0%, #1E5CB3 100%);
-  color: #ffffff;
+  color: var(--color-text-on-accent);
 }
 
 .weather-widget__refresh {
@@ -656,11 +656,11 @@
   height: 28px;
   border: none;
   background: var(--color-glass);
-  color: #fff;
+  color: var(--color-text-on-accent);
   border-radius: var(--radius-full);
   cursor: pointer;
   z-index: 1;
-  transition: background 0.15s;
+  transition: background var(--transition-fast);
 }
 
 .weather-widget__refresh:hover {
@@ -903,7 +903,7 @@
   height: 52px;
   border-radius: var(--radius-full);
   background-color: var(--module-accent, var(--color-accent));
-  color: #ffffff;
+  color: var(--color-text-on-accent);
   box-shadow: var(--shadow-lg);
   display: flex;
   align-items: center;