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/components/ChatMessage.vue

@@ -118,17 +118,16 @@ const timingParts = computed((): string[] => {
 
 .message-bubble {
   padding: 0.75rem 1rem;
-  border-radius: 16px;
+  border-radius: 18px;
 }
 
 .role-user .message-bubble {
-  background: var(--color-primary);
+  background: linear-gradient(135deg, #6366f1, #4f46e5);
   color: #fff;
   border-bottom-right-radius: 4px;
 }
 .role-assistant .message-bubble {
   background: var(--color-bg-card);
-  border: 1px solid var(--color-border);
   border-bottom-left-radius: 4px;
 }