fix(export): split bulk-export toast across success / partial / all-failed

The Phase 9 bulk export utility had a single success toast that was emitted
even when zero of N writes succeeded — "Exported 0 of 5 transcripts to
folder/" reading like the user just deliberately exported nothing.

Branch on the result count:
- 0 of N: error toast pointing at the console for write failures.
- N of N: success toast.
- M of N: warn toast — partial export, with the same console pointer.

Single-file save (`saveTranscriptAsMarkdown`) was already correct:
explicit success on save, error on failure, silent on user-cancelled —
left untouched.

https://claude.ai/code/session_0189xUb6ie6t9qHkzatGZ9Rb

src/lib/utils/saveMarkdown.ts

@@ -90,11 +90,24 @@ export async function exportTranscriptsToDir(
       console.error("exportTranscriptsToDir write failed", path, err);
     }
   }
-  toasts.success(
-    written === items.length
-      ? `Exported ${written} transcript${written === 1 ? "" : "s"} to ${basename(dir)}`
-      : `Exported ${written} of ${items.length} transcripts to ${basename(dir)}`,
-  );
+  if (written === 0) {
+    // All writes failed — surface as an error instead of "Exported 0
+    // transcripts" success toast which reads like the user just bulk-
+    // exported nothing on purpose.
+    toasts.error(
+      "Couldn't export transcripts",
+      `0 of ${items.length} written to ${basename(dir)}. Check the console for details.`,
+    );
+  } else if (written === items.length) {
+    toasts.success(
+      `Exported ${written} transcript${written === 1 ? "" : "s"} to ${basename(dir)}`,
+    );
+  } else {
+    toasts.warn(
+      `Partial export to ${basename(dir)}`,
+      `${written} of ${items.length} transcripts written. Check the console for the failures.`,
+    );
+  }
   return written;
 }