feat: improve graph readability with colored edges, bezier curves, and auto-layout

- Color-code edges by semantic role: structural edges (system↔node) in
  muted zinc, agent-to-agent edges in indigo for clear visual separation
- Switch from smoothstep to bezier curves for smoother, more natural
  edge routing that reduces right-angle overlaps
- Stagger agent nodes at alternating X offsets in concurrent, handoff,
  and group-chat layouts so parallel fan-out/fan-in edges take distinct
  bezier paths instead of stacking
- Increase spacing in all layouts to give edges more room
- Add dagre-powered auto-layout button (top-right of canvas) that
  computes optimal left-to-right hierarchical node positions
- Add 2 new tests for auto-layout across all orchestration modes

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

bun.lock

@@ -5,6 +5,7 @@
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         "@xyflow/react": "^12.10.1",
         "keytar": "^7.9.0",
         "lucide-react": "^0.577.0",
@@ -71,6 +72,10 @@
 
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