Add Save As, auto-migrate v1 projects to folder structure

Save behavior:
- Save on v2 project: saves in place (no dialog)
- Save on v1 project: auto-migrates to folder structure next to the
  original .vtn (creates ProjectName/ folder with .vtn + audio.wav)
- Save on unsaved project: opens folder picker (Save As)
- Save As: always opens folder picker for a new location

Added projectIsV2 state to track project format version.
Split "Save Project" button into "Save" + "Save As".
Extracted saveToFolder() helper for shared save logic.

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

src/routes/+page.svelte

@@ -31,6 +31,7 @@
   // Project management state
   let currentProjectPath = $state<string | null>(null);
   let currentProjectName = $state('');
+  let projectIsV2 = $state(false);
   let audioFilePath = $state('');
   let audioWavPath = $state('');
 
@@ -171,57 +172,65 @@
     };
   }
 
+  /** Save to a specific folder — creates .vtn + audio.wav inside it. */
+  async function saveToFolder(folderPath: string): Promise<boolean> {
+    const projectName = folderPath.split(/[\\/]/).pop() || currentProjectName || 'Untitled';
+    const vtnPath = `${folderPath}/${projectName}.vtn`;
+    const wavPath = `${folderPath}/audio.wav`;
+    const projectData = buildProjectData(projectName);
+
+    try {
+      await invoke('create_dir', { path: folderPath });
+      if (audioWavPath && audioWavPath !== wavPath) {
+        await invoke('copy_file', { src: audioWavPath, dst: wavPath });
+        audioWavPath = wavPath;
+      }
+      await invoke('save_project_file', { path: vtnPath, project: projectData });
+      currentProjectPath = vtnPath;
+      currentProjectName = projectName;
+      projectIsV2 = true;
+      return true;
+    } catch (err) {
+      console.error('Failed to save project:', err);
+      alert(`Failed to save: ${err}`);
+      return false;
+    }
+  }
+
   async function saveProject() {
-    const defaultName = currentProjectName || 'Untitled';
-
-    // If already saved, save in place
-    if (currentProjectPath) {
+    // Already saved as v2 folder — save in place
+    if (currentProjectPath && projectIsV2) {
       const folderPath = currentProjectPath.replace(/[\\/][^\\/]+$/, '');
-      const projectName = currentProjectPath.split(/[\\/]/).pop()?.replace(/\.vtn$/i, '') || defaultName;
-      const wavInsideFolder = `${folderPath}/audio.wav`;
+      await saveToFolder(folderPath);
+      return;
+    }
 
-      const projectData = buildProjectData(projectName);
-      try {
-        if (audioWavPath && audioWavPath !== wavInsideFolder) {
-          await invoke('copy_file', { src: audioWavPath, dst: wavInsideFolder });
-        }
-        await invoke('save_project_file', { path: currentProjectPath, project: projectData });
-      } catch (err) {
-        alert(`Failed to save: ${err}`);
+    // V1 project opened — migrate to folder structure
+    if (currentProjectPath && !projectIsV2) {
+      const oldVtnDir = currentProjectPath.replace(/[\\/][^\\/]+$/, '');
+      const projectName = currentProjectPath.split(/[\\/]/).pop()?.replace(/\.vtn$/i, '') || 'Untitled';
+      const folderPath = `${oldVtnDir}/${projectName}`;
+      const success = await saveToFolder(folderPath);
+      if (success) {
+        // Optionally remove the old .vtn file
+        try {
+          // Leave old file — user can delete manually
+        } catch {}
       }
       return;
     }
 
-    // Pick a folder for the new project
+    // Never saved — pick a folder
+    await saveProjectAs();
+  }
+
+  async function saveProjectAs() {
     const folderPath = await open({
       directory: true,
       title: 'Choose a folder to save the project',
     });
     if (!folderPath) return;
-
-    // Use the folder name as the project name
-    const projectName = (folderPath as string).split(/[\\/]/).pop() || defaultName;
-    const vtnInsideFolder = `${folderPath}/${projectName}.vtn`;
-    const wavInsideFolder = `${folderPath}/audio.wav`;
-
-    const projectData = buildProjectData(projectName);
-
-    try {
-
-      // Copy the extracted WAV into the project folder
-      if (audioWavPath) {
-        await invoke('copy_file', { src: audioWavPath, dst: wavInsideFolder });
-      }
-
-      // Save the .vtn file inside the folder
-      await invoke('save_project_file', { path: vtnInsideFolder, project: projectData });
-
-      currentProjectPath = vtnInsideFolder;
-      currentProjectName = projectName;
-    } catch (err) {
-      console.error('Failed to save project:', err);
-      alert(`Failed to save: ${err}`);
-    }
+    await saveToFolder(folderPath as string);
   }
 
   async function openProject() {
@@ -290,6 +299,7 @@
       // Determine the directory the .vtn file is in
       const vtnDir = (filePath as string).replace(/[\\/][^\\/]+$/, '');
       const version = project.version ?? 1;
+      projectIsV2 = version >= 2;
 
       // Resolve audio for wavesurfer playback
       if (version >= 2) {
@@ -734,7 +744,10 @@
       </button>
       {#if $segments.length > 0}
         <button class="settings-btn" onclick={saveProject}>
-          Save Project
+          Save
+        </button>
+        <button class="settings-btn" onclick={saveProjectAs}>
+          Save As
         </button>
       {/if}
       <button class="import-btn" onclick={handleFileImport} disabled={isTranscribing}>