fix: Docker status never updates after Docker starts

Replace OnceLock with Mutex<Option<Docker>> in the Rust backend so
failed Docker connections are retried instead of cached permanently.
Add frontend polling (every 5s) when Docker is initially unavailable,
stopping once detected.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src/hooks/useDocker.ts

@@ -1,4 +1,4 @@
-import { useCallback } from "react";
+import { useCallback, useRef } from "react";
 import { useShallow } from "zustand/react/shallow";
 import { listen } from "@tauri-apps/api/event";
 import { useAppState } from "../store/appState";
@@ -59,6 +59,39 @@ export function useDocker() {
     [setImageExists],
   );
 
+  const pollingRef = useRef<ReturnType<typeof setInterval> | null>(null);
+
+  const startDockerPolling = useCallback(() => {
+    // Don't start if already polling
+    if (pollingRef.current) return () => {};
+
+    const interval = setInterval(async () => {
+      try {
+        const available = await commands.checkDocker();
+        if (available) {
+          clearInterval(interval);
+          pollingRef.current = null;
+          setDockerAvailable(true);
+          // Also check image once Docker is available
+          try {
+            const exists = await commands.checkImageExists();
+            setImageExists(exists);
+          } catch {
+            setImageExists(false);
+          }
+        }
+      } catch {
+        // Still not available, keep polling
+      }
+    }, 5000);
+
+    pollingRef.current = interval;
+    return () => {
+      clearInterval(interval);
+      pollingRef.current = null;
+    };
+  }, [setDockerAvailable, setImageExists]);
+
   const pullImage = useCallback(
     async (imageName: string, onProgress?: (msg: string) => void) => {
       const unlisten = onProgress
@@ -84,5 +117,6 @@ export function useDocker() {
     checkImage,
     buildImage,
     pullImage,
+    startDockerPolling,
   };
 }