Fix security and correctness issues: auth bypass, PIN detection, network drift, timer leak

[Almikefred]

Summary

Resolves four security and correctness issues across ZKP service, wallet recovery, network detection, and auth token management:

• Removes unauthenticated access to proof listings
• Fixes browser-incompatible error detection
• Consolidates duplicated network detection logic
• Prevents token-refresh interval leaks from concurrent timers

Changes

#708 — zkp.ts getProofsForPet bypasses auth

• Create /api/zkp/pet/[petId] route to proxy backend with authentication
• Update zkpService.getProofsForPet() to route through internal API
• Attaches auth token consistent with generateProof and verifyProof

#709 — WalletRecovery PIN detection relies on fragile string matching

• Create DecryptionError class for typed error handling
• Update decryptSecretKey() to wrap Web Crypto API failures
• Replace substring matching with instanceof check in WalletRecovery
• Works reliably across all browsers/engines

#710 — Network detection logic duplicated in 3 places

• Extract getStellarNetwork() and isTestnetNetwork() to src/lib/blockchain/network.ts
• Update walletService, blockchain/index, and wallet.tsx to use shared helper
• Single source of truth eliminates drift risk

#711 — AuthContext token-refresh timer stored in closure variable (HIGH priority)

• Move refreshTimer from plain let to useRef for cross-render persistence
• Add useEffect cleanup to clear interval on unmount
• Prevents duplicate/racing refresh calls from orphaned intervals

Closes #708, Closes #709, Closes #710, Closes #711