feat: implement issues #58, #59, #60, #61

[katefominmoore23]

#58 docs: add docs/error-codes.md with all InvoiceError and SettlementError codes, numeric values, trigger conditions, remediation steps, and HTTP status mapping table.

#59 feat: add NetworkSelector component to dashboard header

• useNetwork hook with localStorage persistence (comebackhere-network key)
• NetworkSelector dropdown (testnet/mainnet) with mainnet warning banner
• DashboardLayout header updated to render NetworkSelector alongside theme toggle

#60 feat: add POST /disputes endpoint to comebackhere-backend

• disputes.ts route validates claimant_address (Stellar pubkey) and settlement_id
• transitions settlement to OnHold and returns dispute_id + status
• registered at /disputes in app.ts

#61 feat: add InvoiceSearchFilter component to invoices page

• search by invoice ID or merchant Stellar address
• filter by InvoiceStatus enum (Pending/Paid/Expired/Cancelled/RefundRequested/Released)
• date range picker for created_at filtering
• paginated results with configurable page size (10/25/50)
• InvoiceStatus and Invoice types added to frontend/src/types/index.ts
• InvoicesPage in App.tsx wired up to InvoiceSearchFilter

Closes #58
Closes #59
Closes #60
Closes #61

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