useNetwork hook and Spacescan testnet links

[dkackman]

This refactors mainnet/testnet url creation into a single urls file and a useNetwork hook, centralizing both rather than having url construction and testnet determination scattered through the fronend.

[dkackman]

Code review

Found 2 issues:

1. ** always uses the mainnet Dexie API, even on testnet** — the function hardcodes false for isTestnet, while the sibling executeDexieSwap in the same file correctly threads the actual isTestnet value. On testnet, users receive mainnet swap quotes but submit testnet swaps, producing a price/liquidity mismatch.

sage/src/pages/Swap.tsx

Lines 370 to 385 in d9f74d9

	receiveAssetId: string | null,
	amount: string,
	amountKind: 'pay' | 'receive',
	) {
	try {
	const response = await fetch(
	dexieApiUrl(
	`v1/swap/quote?from=${payAssetId ?? 'XCH'}&to=${receiveAssetId ?? 'XCH'}&${amountKind === 'pay' ? 'from_amount' : 'to_amount'}=${amount || '0'}`,
	false,
	),
	);
	const data = await response.json();
	return {
	amount: (amountKind === 'pay'
	? data.quote.to_amount
	: data.quote.from_amount) as number,

2. OfferCreationProgressDialog can upload offers to the wrong network during the loading window — the network && guard that prevented uploads while network state was unknown was removed. useNetwork initializes network as null and derives isTestnet: network !== 'mainnet', so isTestnet is true on the first render before getNetwork() resolves. If an upload triggers in that window on mainnet, it hits testnet marketplace endpoints. The guard should be restored as network !== null using the network value returned alongside isTestnet.

sage/src/hooks/useNetwork.ts

Lines 19 to 22 in d9f74d9

	});
	}, []);
	return { network, isTestnet: network !== 'mainnet' };

sage/src/components/dialogs/OfferCreationProgressDialog.tsx

Lines 73 to 77 in d9f74d9

	// Handle uploads when offers are created
	useEffect(() => {
	if (createdOffers.length > 0 && !isProcessing && !isCanceling) {
	let isMounted = true;

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