UI: indigo/slate theme, border cleanup, rounder edges

theme.css:
- Dark mode palette shifted from navy to dark slate with indigo warmth
  (#111113 / #18181f / #1e1e27). Primary restored to true indigo
  (#818cf8) instead of the blue (#5b9cf6) it had drifted to.
- Borders reduced to near-invisible rgba(99,102,241,0.10) indigo tint.
- --radius-md 8px→12px, --radius-lg 12px→18px.
- Tag/wikilink/status-in-progress colors unified to indigo family.
- Thin indigo-tinted scrollbars (4px, rgba thumb).

ChatMessage.vue:
- Message bubbles 16px→18px radius.
- User bubble: flat primary → indigo gradient (#6366f1→#4f46e5).
- Assistant bubble: border removed, relies on bg-card vs bg contrast.

ChatView.vue:
- Sidebar border-right, chat-header border-bottom, context-sidebar
  border-left all removed.
- Streaming bubble: border removed, radius 16px→18px.
- Active conv item: solid primary fill → 15% opacity indigo tint with
  primary-colored text (softer, more refined selection state).
- Send button: flat primary → indigo gradient + glow on hover.
- New Chat button: glow on hover.
- Queued bubble: matches send button gradient.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/ChatView.vue

@@ -847,7 +847,6 @@ onUnmounted(() => {
 .chat-sidebar {
   width: 260px;
   min-width: 200px;
-  border-right: 1px solid var(--color-border);
   display: flex;
   flex-direction: column;
   background: var(--color-bg-secondary);
@@ -868,9 +867,11 @@ onUnmounted(() => {
   border-radius: var(--radius-sm);
   cursor: pointer;
   font-size: 0.9rem;
+  transition: box-shadow 0.15s, opacity 0.15s;
 }
 .btn-new-conv:hover {
   opacity: 0.9;
+  box-shadow: 0 0 14px rgba(129, 140, 248, 0.35);
 }
 
 .btn-select-mode {
@@ -996,8 +997,8 @@ onUnmounted(() => {
   background: var(--color-bg-card);
 }
 .conv-item.active {
-  background: var(--color-primary);
-  color: #fff;
+  background: color-mix(in srgb, var(--color-primary) 15%, transparent);
+  color: var(--color-primary);
 }
 .conv-info {
   flex: 1;
@@ -1017,7 +1018,7 @@ onUnmounted(() => {
   margin-top: 1px;
 }
 .conv-item.active .conv-date {
-  color: rgba(255, 255, 255, 0.7);
+  color: var(--color-text-muted);
 }
 .btn-delete-conv {
   background: none;
@@ -1029,7 +1030,7 @@ onUnmounted(() => {
   line-height: 1;
 }
 .conv-item.active .btn-delete-conv {
-  color: rgba(255, 255, 255, 0.7);
+  color: var(--color-text-muted);
 }
 .btn-delete-conv:hover {
   color: var(--color-danger, #e74c3c);
@@ -1047,7 +1048,6 @@ onUnmounted(() => {
   align-items: center;
   justify-content: space-between;
   padding: 0.75rem 1rem;
-  border-bottom: 1px solid var(--color-border);
 }
 .chat-header h2 {
   margin: 0;
@@ -1116,7 +1116,6 @@ onUnmounted(() => {
 .context-sidebar {
   width: 220px;
   min-width: 180px;
-  border-left: 1px solid var(--color-border);
   background: var(--color-bg-secondary);
   display: flex;
   flex-direction: column;
@@ -1220,10 +1219,9 @@ onUnmounted(() => {
 .streaming-bubble {
   max-width: 80%;
   padding: 0.75rem 1rem;
-  border-radius: 16px;
+  border-radius: 18px;
   border-bottom-left-radius: 4px;
   background: var(--color-bg-card);
-  border: 1px solid var(--color-border);
 }
 .streaming-bubble .message-header {
   display: flex;
@@ -1517,13 +1515,17 @@ details[open] .thinking-summary::before {
   display: flex;
   align-items: center;
   justify-content: center;
-  background: var(--color-primary);
+  background: linear-gradient(135deg, #6366f1, #4f46e5);
   color: #fff;
   border: none;
   border-radius: 50%;
   cursor: pointer;
   font-size: 1.1rem;
   flex-shrink: 0;
+  transition: box-shadow 0.15s;
+}
+.btn-send:not(:disabled):hover {
+  box-shadow: 0 0 14px rgba(129, 140, 248, 0.5);
 }
 .btn-send:disabled {
   opacity: 0.35;
@@ -1628,9 +1630,9 @@ details[open] .thinking-summary::before {
 .queued-bubble {
   max-width: 80%;
   padding: 0.75rem 1rem;
-  border-radius: 16px;
+  border-radius: 18px;
   border-bottom-right-radius: 4px;
-  background: var(--color-primary);
+  background: linear-gradient(135deg, #6366f1, #4f46e5);
   color: #fff;
   opacity: 0.45;
   font-size: 0.95rem;