style(knowledge): align task card theming with note/list pattern

Task card was the outlier in the Knowledge view: half-width fade-to-
transparent gradient bar while note/list use full-width two-stop
gradients, and the type badge was purple while the border+bar were gold.

- k-card--task::before: full width with gold→amber gradient (#d4a017
  → #fbbf24) matching the two-stop pattern
- badge--task: switched from purple to amber so the badge matches
  the card chrome

frontend/src/views/KnowledgeView.vue

@@ -1025,8 +1025,8 @@ onUnmounted(() => {
   background: linear-gradient(90deg, #7c3aed, #a78bfa);
 }
 .k-card--task::before {
-  width: 50%;
-  background: linear-gradient(90deg, #d4a017, transparent);
+  right: 0;
+  background: linear-gradient(90deg, #d4a017, #fbbf24);
 }
 .k-card--list::before {
   right: 0;
@@ -1064,7 +1064,7 @@ onUnmounted(() => {
 .badge--person { background: rgba(16,185,129,0.15); color: #34d399; }
 .badge--place  { background: rgba(245,158,11,0.15); color: #fbbf24; }
 .badge--list   { background: rgba(56,189,248,0.15); color: #7dd3fc; }
-.badge--task   { background: rgba(167,139,250,0.15); color: #a78bfa; }
+.badge--task   { background: rgba(212,160,23,0.15); color: #fbbf24; }
 
 .k-card-body { flex: 1; padding-right: 40px; }
 .k-card-title {