feat: add save diagram to local file button (#60)

- Add save button in chat input area with download icon
- Create SaveDialog component for filename input
- Export current diagram as .drawio file format
- Support custom filename with default timestamp-based name

Closes #53

components/save-dialog.tsx

@@ -0,0 +1,80 @@
+"use client";
+
+import { useState, useEffect } from "react";
+import { Button } from "@/components/ui/button";
+import { Input } from "@/components/ui/input";
+import {
+    Dialog,
+    DialogContent,
+    DialogHeader,
+    DialogTitle,
+    DialogFooter,
+} from "@/components/ui/dialog";
+
+interface SaveDialogProps {
+    open: boolean;
+    onOpenChange: (open: boolean) => void;
+    onSave: (filename: string) => void;
+    defaultFilename: string;
+}
+
+export function SaveDialog({
+    open,
+    onOpenChange,
+    onSave,
+    defaultFilename,
+}: SaveDialogProps) {
+    const [filename, setFilename] = useState(defaultFilename);
+
+    useEffect(() => {
+        if (open) {
+            setFilename(defaultFilename);
+        }
+    }, [open, defaultFilename]);
+
+    const handleSave = () => {
+        const finalFilename = filename.trim() || defaultFilename;
+        onSave(finalFilename);
+        onOpenChange(false);
+    };
+
+    const handleKeyDown = (e: React.KeyboardEvent) => {
+        if (e.key === "Enter") {
+            e.preventDefault();
+            handleSave();
+        }
+    };
+
+    return (
+        <Dialog open={open} onOpenChange={onOpenChange}>
+            <DialogContent className="sm:max-w-md">
+                <DialogHeader>
+                    <DialogTitle>Save Diagram</DialogTitle>
+                </DialogHeader>
+                <div className="space-y-2">
+                    <label className="text-sm font-medium">Filename</label>
+                    <div className="flex items-stretch">
+                        <Input
+                            value={filename}
+                            onChange={(e) => setFilename(e.target.value)}
+                            onKeyDown={handleKeyDown}
+                            placeholder="Enter filename"
+                            autoFocus
+                            onFocus={(e) => e.target.select()}
+                            className="rounded-r-none border-r-0 focus-visible:z-10"
+                        />
+                        <span className="inline-flex items-center px-3 rounded-r-md border border-l-0 border-input bg-muted text-sm text-muted-foreground font-mono">
+                            .drawio
+                        </span>
+                    </div>
+                </div>
+                <DialogFooter>
+                    <Button variant="outline" onClick={() => onOpenChange(false)}>
+                        Cancel
+                    </Button>
+                    <Button onClick={handleSave}>Save</Button>
+                </DialogFooter>
+            </DialogContent>
+        </Dialog>
+    );
+}