feat: DAG visualization for trajectory page with agent/terminal support

Replace flat timeline with top-to-bottom DAG using SVG bezier edges.
Add parallel step bands, collapse/expand, hover path highlighting.
New step types: agent_call and terminal with metadata display.
Edge anchors connect from closest node edge, not center.
Duration shown compactly in header row right corner.

Author: zhmzm

packages/web/src/index.css

@@ -50,12 +50,18 @@
   --c-curve-hi: #94A3B8;
   --c-curve-lo: #E5E7EB;
 
+  /* DAG */
+  --c-dag-bg: #FAFBFC;
+  --c-dag-node-shadow: rgba(0,0,0,0.04);
+
   /* Trajectory step types */
   --c-step-tool-bg: #EFF6FF; --c-step-tool-border: #BFDBFE; --c-step-tool-text: #1D4ED8;
   --c-step-decision-bg: #F5F3FF; --c-step-decision-border: #DDD6FE; --c-step-decision-text: #6D28D9;
   --c-step-observation-bg: #F8FAFC; --c-step-observation-border: #E2E8F0; --c-step-observation-text: #64748B;
   --c-step-error-bg: #FEF2F2; --c-step-error-border: #FECACA; --c-step-error-text: #B91C1C;
   --c-step-retry-bg: #FFFBEB; --c-step-retry-border: #FDE68A; --c-step-retry-text: #A16207;
+  --c-step-agent-bg: #F0FDF4; --c-step-agent-border: #86EFAC; --c-step-agent-text: #15803D;
+  --c-step-terminal-bg: #F8FAFC; --c-step-terminal-border: #CBD5E1; --c-step-terminal-text: #334155;
 }
 
 .dark {
@@ -106,12 +112,18 @@
   --c-curve-hi: #636366;
   --c-curve-lo: #2C2C2E;
 
+  /* DAG */
+  --c-dag-bg: #151516;
+  --c-dag-node-shadow: rgba(0,0,0,0.3);
+
   /* Trajectory step types */
   --c-step-tool-bg: #001933; --c-step-tool-border: #003A80; --c-step-tool-text: #0A84FF;
   --c-step-decision-bg: #1A0533; --c-step-decision-border: #3B0764; --c-step-decision-text: #A78BFA;
   --c-step-observation-bg: #1C1C1E; --c-step-observation-border: #3A3A3C; --c-step-observation-text: #98989E;
   --c-step-error-bg: #2B1C1C; --c-step-error-border: #6B2020; --c-step-error-text: #FF453A;
   --c-step-retry-bg: #2B2218; --c-step-retry-border: #6B4D10; --c-step-retry-text: #FF9F0A;
+  --c-step-agent-bg: #1C2B22; --c-step-agent-border: #1D5C38; --c-step-agent-text: #30D158;
+  --c-step-terminal-bg: #1C1C1E; --c-step-terminal-border: #3A3A3C; --c-step-terminal-text: #98989E;
 }
 
 body {