fix: TS2345 null check on canvas context in RDP frame loop

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/composables/useRdp.ts

@@ -322,6 +322,7 @@ export function useRdp(): UseRdpReturn {
         rafScheduled = false;
         if (fetchPending) return;
         fetchPending = true;
+        if (!ctx) return;
         const rendered = await fetchAndRender(sessionId, width, height, ctx);
         fetchPending = false;
         if (rendered && !connected.value) connected.value = true;