Add Docker install helper for first-run setup

When Docker isn't detected on startup, surface a dialog offering a
one-click install (pkexec + get.docker.com on Linux, brew cask on
macOS, winget on Windows) with a graceful fallback to manual steps
and a link to official documentation. Install output streams back
to the UI via a tauri event.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/src/hooks/useInstallHelper.ts

@@ -0,0 +1,35 @@
+import { useCallback, useState } from "react";
+import { listen } from "@tauri-apps/api/event";
+import * as commands from "../lib/tauri-commands";
+import type { InstallOptions } from "../lib/types";
+
+export function useInstallHelper() {
+  const [options, setOptions] = useState<InstallOptions | null>(null);
+
+  const loadOptions = useCallback(async () => {
+    try {
+      const opts = await commands.detectInstallOptions();
+      setOptions(opts);
+      return opts;
+    } catch {
+      setOptions(null);
+      return null;
+    }
+  }, []);
+
+  const runInstall = useCallback(
+    async (onProgress?: (line: string) => void) => {
+      const unlisten = onProgress
+        ? await listen<string>("docker-install-progress", (e) => onProgress(e.payload))
+        : null;
+      try {
+        await commands.runDockerInstall();
+      } finally {
+        unlisten?.();
+      }
+    },
+    [],
+  );
+
+  return { options, loadOptions, runInstall };
+}