fix(walletapi): persist network setting and reset Connected on connectivity failure

[moralpriest]

fix(walletapi): persist wallet network setting in JSON

Problem:
The mainnet field in the Account struct was lowercase (unexported),
which caused json.Marshal() to skip it during wallet serialization.
When wallets were saved and reopened, the network setting defaulted to
false (testnet) regardless of what the wallet was originally created
with.
Impact:

• Wallets created as mainnet showed testnet addresses (deto1...)
  after reopening
• Simulator wallets failed with "Mainnet/TestNet mismatch" because
  the wallet reverted to testnet format
• Tools like TELA and Engram had to implement workarounds calling
  SetNetwork() after every wallet open
• Address formatting was inconsistent across sessions
  Root Cause:
  Go's encoding/json only serializes exported (uppercase) fields.
  The field mainnet bool was lowercase, so it was invisible to JSON.
  Fix:
  Changed mainnet to Mainnet with json:"mainnet" tag so the
  network setting is properly persisted across save/load cycles.
  Files Changed:
• walletapi/wallet.go:
  • Account.Mainnet field (exported with JSON tag)
  • GetAddress() reference updated
  • SetNetwork() reference updated
  • GetNetwork() reference updated
    Backward Compatibility:
• Old wallets without "mainnet" in JSON: default to false (testnet)
  • This preserves existing behavior for old wallets
  • Tools can call SetNetwork() to upgrade if needed
• New wallets: correctly persist mainnet=true/false
  Testing:
• Created wallet with mainnet simulator setting
• Saved wallet to disk
• Reopened wallet: address correctly shows dero1... (mainnet)
• Without this fix: address would show deto1... (testnet)
  Related Issues:
• Fixes simulator wallet connection failures
• Removes need for TELA workaround (SetNetwork override)

Note: Pre-existing test failures (Test_Payload_TX, Test_Creation_TX_morecheck) are unrelated to this change and present on base branch community-dev.

[moralpriest]

Updated this PR with an additional fix:

Problem: `walletapi.Connect()` sets `Connected=true` immediately after a successful WebSocket dial, but then returns directly from `test_connectivity()` without resetting it if that subsequent check fails.

This causes consumers (e.g., Engram) to observe `walletapi.Connected == true` even when the daemon connection is actually invalid — such as when a mainnet/testnet network mismatch is detected, or when `DERO.Echo`/`DERO.GetInfo` RPC calls fail. Downstream code then incorrectly proceeds with wallet registration PoW and other background tasks against a broken connection, wasting CPU and producing confusing UI state.

Fix: In `daemon_connectivity.go`, reset `Connected = false` explicitly before returning from `Connect()` when `test_connectivity()` returns an error.

This change is minimal, defensive, and preserves the existing happy path. Both fixes are in `walletapi/daemon_connectivity.go` and are ready for review.