Save As: use save dialog so user can type a new project name

Changed from folder picker (can only select existing folders) to save
dialog where the user can type a new name. The typed name becomes the
project folder, created automatically if it doesn't exist. Any file
extension the user types is stripped (e.g. "My Project.vtn" becomes
the folder "My Project/").

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

src/routes/+page.svelte

@@ -225,12 +225,18 @@
   }
 
   async function saveProjectAs() {
-    const folderPath = await open({
-      directory: true,
-      title: 'Choose a folder to save the project',
+    // Use save dialog so the user can type a new project name.
+    // The chosen path is treated as the project folder (created if needed).
+    const defaultName = currentProjectName || 'Untitled';
+    const chosenPath = await save({
+      defaultPath: defaultName,
+      title: 'Save Project — enter a project name',
     });
-    if (!folderPath) return;
-    await saveToFolder(folderPath as string);
+    if (!chosenPath) return;
+
+    // Strip any file extension the user may have typed (e.g. ".vtn")
+    const folderPath = chosenPath.replace(/\.[^.\\/]+$/, '');
+    await saveToFolder(folderPath);
   }
 
   async function openProject() {