Batch mode: synthesize a folder of text files

Creating many voiceovers one-by-one is tedious.

**Request:** A batch option that takes a folder of .txt/.md files and synthesizes each to its own output file (same format setting, same or per-file voice), with a progress list.

**Acceptance criteria:** selecting a folder synthesizes every file, preserves filenames (e.g. ile1.txt -> ile1.mp3), and reports per-file status.