feat(export): prefer native save dialog over browser blob fallback

DictationPage + FilesPage handleExport() now use Tauri save() +
write_text_file_cmd when in the desktop runtime; browser blob path
remains as fallback when hasTauriRuntime() is false. saveMarkdown.ts
surfaces dialog errors via toasts. Adds txt to the extension map.

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

src/lib/utils/saveMarkdown.ts

@@ -33,14 +33,24 @@ export function suggestedFilename(item: TranscriptEntry): string {
 export async function saveTranscriptAsMarkdown(
   item: TranscriptEntry,
 ): Promise<string | null> {
-  if (!hasTauriRuntime()) return null;
+  if (!hasTauriRuntime()) {
+    toasts.error("Export unavailable", "Native save dialogs are only available in the desktop app.");
+    return null;
+  }
   const md = buildMarkdown(item);
   const defaultPath = suggestedFilename(item);
-  const path = await save({
-    title: "Save transcript as Markdown",
-    defaultPath,
-    filters: [{ name: "Markdown", extensions: ["md"] }],
-  });
+  let path: string | null = null;
+  try {
+    path = await save({
+      title: "Save transcript as Markdown",
+      defaultPath,
+      filters: [{ name: "Markdown", extensions: ["md"] }],
+    });
+  } catch (err) {
+    console.error("saveTranscriptAsMarkdown dialog failed", err);
+    toasts.error("Couldn't open save dialog", String(err));
+    return null;
+  }
   if (!path) return null;
   try {
     await invoke("write_text_file_cmd", { path, contents: md });
@@ -64,13 +74,24 @@ export async function saveTranscriptAsMarkdown(
 export async function exportTranscriptsToDir(
   items: TranscriptEntry[],
 ): Promise<number> {
-  if (!hasTauriRuntime() || items.length === 0) return 0;
+  if (items.length === 0) return 0;
+  if (!hasTauriRuntime()) {
+    toasts.error("Export unavailable", "Native folder dialogs are only available in the desktop app.");
+    return 0;
+  }
 
-  const dir = await open({
-    directory: true,
-    multiple: false,
-    title: "Choose export folder",
-  });
+  let dir: string | string[] | null = null;
+  try {
+    dir = await open({
+      directory: true,
+      multiple: false,
+      title: "Choose export folder",
+    });
+  } catch (err) {
+    console.error("exportTranscriptsToDir dialog failed", err);
+    toasts.error("Couldn't open export folder dialog", String(err));
+    return 0;
+  }
   if (!dir || typeof dir !== "string") return 0;
 
   const used = new Set<string>();