fix: flush pending debounced localStorage saves on pagehide in standalone dev harness

[Copilot]

Addresses review comments from #pullrequestreview-4656536637 on PR #1423 (autosave dev sources in localStorage).

Changes

packages/standalone/src/test-main.tsx

• Adds a pendingSources: Map<string, string> to track writes scheduled but not yet persisted
• Adds a pagehide listener that cancels pending timers and flushes all pending sources synchronously — mirrors the identical pattern already used in doenetml-iframe/src/test-main.tsx
• saveSource() now tracks/clears entries in pendingSources alongside saveTimers

window.addEventListener("pagehide", () => {
    for (const [key, source] of pendingSources) {
        const timer = saveTimers.get(key);
        if (timer !== undefined) {
            window.clearTimeout(timer);
            saveTimers.delete(key);
        }
        try {
            localStorage.setItem(key, source);
            pendingSources.delete(key);
        } catch { /* ignore */ }
    }
    pendingSources.clear();
});

packages/doenetml-iframe/src/test-main.tsx

• Changes saveTimer type from number | null → ReturnType<typeof window.setTimeout> | null for consistency with the standalone implementation and correctness when Node types are in scope