feat: flow copy/paste with undo/redo support

[moosebay]

Summary

• Copy/paste flow nodes (Cmd+C / Cmd+V) with full type-specific data preservation via YAML serialization, position offsets, variable remapping, and edge reconstruction between copied nodes
• Undo/redo (Cmd+Z / Cmd+Shift+Z / Cmd+Y) for canvas operations: position drag, node delete, edge delete, edge create, and paste
• Smart paste positioning: same-flow paste offsets 200px below selection, cross-flow paste centers in viewport
• Extract useFlowSelection hook for centralized selection management (also used in agent panel)
• Remove OpenReplay integration
• Bump @xyflow/react to 12.10.1 (fixes drag/selection bugs)

New server RPCs

• FlowNodesCopy — serializes selected nodes to YAML (including HTTP config, JS code, conditions, AI settings)
• FlowNodesPaste — deserializes YAML back into new nodes with fresh IDs, variable remapping, and edge reconstruction

Test plan

• Server copy/paste tests (rflowv2_copy_paste_test.go)
• Copy nodes → paste in same flow → nodes appear 200px below with correct data
• Copy nodes → paste in different flow → nodes appear at viewport center
• Drag node → Cmd+Z → returns to original position → Cmd+Shift+Z → goes back
• Delete node → Cmd+Z → node reappears with correct type-specific data
• Delete edge → Cmd+Z → edge reappears
• Create edge → Cmd+Z → edge removed
• Paste nodes → Cmd+Z → pasted nodes removed
• Multi-node drag → single Cmd+Z reverts all
• Typing in input/textarea → Cmd+Z does browser undo, not canvas undo