feat: Payment Path Simulator - route finding, hop visualization, slippage estimation

[devboluwatife53]

closes #20

Full implementation of the Payment Path Simulator feature for the Stellar developer workstation.

Changes

Backend (apps/api/src/modules/simulator/)

• GET /simulator/paths — wraps Horizon /paths/strict-send and /paths/strict-receive with per-hop asset breakdown, effective exchange rate, estimated fee, and recommended slippage based on order book depth
• POST /simulator/estimate — given a selected path and slippage tolerance %, returns computed destination_min (strict send) or send_max (strict receive)
• DTOs with class-validator validation for both endpoints
• SimulatorModule registered in app.module.ts

Frontend (apps/web/src/components/tools/simulator-tool.tsx)

• Asset pickers — XLM toggle + token code/issuer inputs for source and destination
• Direction toggle — "Send exactly" (strict_send) vs "Receive exactly" (strict_receive)
• Hop chain visualization — asset bubbles connected by arrows (up to 6 hops)
• Results panel — effective rate, hop count, recommended slippage badge per path
• Slippage tolerance input (%) — instant client-side recalculation of destination_min/send_max (no API call)
• "Use in Composer" button — navigates to /composer pre-filled with all path payment fields
• Empty state — explains why no paths were found (illiquid pair, wrong issuer, etc.)
• Network toggle — works on both testnet and mainnet

Acceptance Criteria

• Paths returned match Horizon's response for the same query
• Hop chain renders correctly for paths up to 6 hops
• Slippage recalculation is instant — purely client-side
• "Use in Composer" correctly pre-populates all path payment fields including intermediate assets
• Empty state clearly explains why no paths were found
• Works on both testnet and mainnet via the global network toggle