test(SorokitProvider): cover switchNetwork state reset; close stale resolved issues

[uche001-dev]

Summary

Re-verified #523, #522, and #519 on current main — all three are genuinely fixed already, by earlier work (#611, merged; #640, open), which disclosed the fixes via comments instead of closing keywords, so all three issues are stuck open despite being resolved:

• Network switching in main.tsx adapter always returns current testnet config regardless of selection #523 (network switching always returns current config) — switchNetwork in src/context/SorokitProvider.tsx genuinely switches networks: it calls client.network.switchNetwork(param), re-points the getClient() singleton at the new network via createClientForNetwork, persists the selection to localStorage, and clears stale address/account/balance state. Confirmed intact.
• Client initialization crash in main.tsx happens before ErrorBoundary renders #522 (client init crash before ErrorBoundary) — src/main.tsx creates the client lazily via useState(createClient) inside a Root component that's already wrapped in <ErrorBoundary>, so a failed initialization is caught by React's error boundary instead of crashing before render() runs. Confirmed intact.
• QRCode renders a fake pixel pattern instead of a real scannable QR code #519 (fake QR code placeholder) — src/components/QRCode.tsx uses the real qrcode library (QRCodeLib.toCanvas), with loading/error states and an accessible aria-label. Confirmed intact.

What this PR actually adds

While re-verifying #523, I noticed SorokitProvider.test.tsx covers switchNetwork updating network state and re-pointing the client singleton, but nothing asserts that switching networks clears the previous network's address, account, and balances — even though the provider's switchNetwork explicitly resets all three on a successful switch. Without that test, a regression here (e.g. someone removing the reset while refactoring) would silently show a user's mainnet balances while the UI reports they're on testnet, with no test catching it.

Added one test, switchNetwork clears stale address, account, and balances from the previous network (#523), following the existing disconnectWallet clears... test's pattern: connect, populate account/balances, switch network, assert all three reset to empty.

Not touching the three fixes themselves — they don't need it.

Verification

• npx vitest run src/context/SorokitProvider.test.tsx — 34/34 passing (was 33/33; the new test is additive).
• npx eslint src/context/SorokitProvider.test.tsx — clean.
• npx tsc --noEmit — clean across the whole project.

Why this PR

These three issues are done in every sense except GitHub's bookkeeping. This PR carries the closing keywords the earlier PRs deliberately omitted (per their own disclosure comments), while adding one small, real piece of test coverage surfaced by reviewing the fix — not a no-op housekeeping PR.

closes #523
closes #522
closes #519

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