fix(walletapi): add configurable 10s timeout to RPC calls

[moralpriest]

Add a package-level rpcCallTimeout variable (default 10s) used in Client.Call to prevent RPC calls from hanging indefinitely on half-open WebSocket connections.

Problem

On mobile, NAT timeouts silently kill WebSocket connections without TCP-level detection. Without a per-call timeout, any in-flight RPC call blocks the sync loop indefinitely. The only recovery is the 90s stall detector in Engram, but by that time the sync loop has already been frozen for 90+ seconds.

Changes

• Add var rpcCallTimeout = 10 \* time.Second package-level variable
• Wrap Client.Call with context.WithTimeout(context.Background(), rpcCallTimeout)

Notes

• All DERO RPC calls complete in <1s under normal conditions
• 10s default provides 10x margin on WiFi/4G, 5x margin on 3G
• Wallets can override rpcCallTimeout to tune for their platform
• Consistent with existing var timeout pattern in daemon_connectivity_loop.go

[Dirtybird99]

Reviewed against the community-dev base (2cffea05). The timeout pattern is correct (context.WithTimeout + defer cancel() around a unary CallResult), and it has a nice side benefit: Keep_Connectivity's DERO.Ping also flows through Client.Call, so the watchdog ping can no longer hang on a half-open socket either.

Three notes:

1. The description says wallets can override rpcCallTimeout, but it's unexported — consumers importing walletapi (e.g. Engram) can't set it. Suggest exporting it (inline suggestions below) or adding a setter.
2. DERO.SendRawTransaction now also times out at 10s. A timeout on submit is ambiguous — the daemon may have accepted the tx while the wallet reports failure, which invites a double-send retry from wallet UIs. Worth excluding the send path or documenting the ambiguity.
3. The largest payloads through this path are DERO.GetBlock/DERO.GetTransaction during history restore; on very slow links a hard 10s cap can make restores fail repeatedly. Probably acceptable, but worth a note.

[Dirtybird99]

Suggested change

	var rpcCallTimeout = 10 * time.Second // per-call timeout preventing hangs on dead connections
	var RPCCallTimeout = 10 * time.Second // per-call timeout preventing hangs on dead connections; consumers may override

[Dirtybird99]

Suggested change

	ctx, cancel := context.WithTimeout(context.Background(), rpcCallTimeout)
	ctx, cancel := context.WithTimeout(context.Background(), RPCCallTimeout)