feat: optimistic message rendering with send status indicators

- User messages appear instantly with 'sending' state (dimmed, clock icon)
- Transitions to 'sent' (checkmark) when server acknowledges
- Shows error state (alert icon, retry visible) if send fails
- Applied to both primary and secondary sessions

src/index.css

@@ -39,8 +39,6 @@
   --pc-accent-dim: rgba(34,211,238,0.3);
   --pc-accent-glow: rgba(34,211,238,0.1);
   --pc-accent-rgb: 34,211,238;
-  --pc-user-bubble: rgba(34,211,238,0.06);
-  --pc-user-border: rgba(34,211,238,0.15);
   --pc-hover: rgba(255,255,255,0.05);
   --pc-hover-strong: rgba(255,255,255,0.08);
   --pc-separator: rgba(255,255,255,0.05);