refactor: unify duplicated logic between WebUI and PyQt

domain/output_paths.py:
- Add sanitize_filename_for_chapter() with OS safety + smart truncation
- Keep existing slugify() for backward compatibility

domain/text_chapters.py (NEW):
- parse_chapters_from_text() combines intro preservation (PyQt) + clean_text (WebUI)

PyQt/conversion.py:
- Replace 3x copy-pasted ASS/SRT headers with create_subtitle_writer()
- Replace inline M4B muxing with ExportService.embed_m4b_metadata()
- Replace inline chapter splitting with parse_chapters_from_text()
- Replace inline chapter filename sanitization with sanitize_filename_for_chapter()
- Remove unused _CHAPTER_MARKER_SEARCH_PATTERN import

Tests: 1152 passed (+21 new)

tests/test_domain_sanitize_filename.py

@@ -0,0 +1,49 @@
+"""Tests for sanitize_filename_for_chapter in domain/output_paths.py"""
+
+from __future__ import annotations
+
+from abogen.domain.output_paths import sanitize_filename_for_chapter
+
+
+class TestSanitizeFilenameForChapter:
+    def test_basic_title(self):
+        result = sanitize_filename_for_chapter("The Beginning", 1)
+        assert result == "01_The_Beginning"
+
+    def test_special_chars_removed(self):
+        result = sanitize_filename_for_chapter("Ch. 1: Hello!", 2)
+        assert result.startswith("02_")
+        assert "Ch" in result
+        assert "Hello" in result
+
+    def test_empty_title_uses_fallback(self):
+        result = sanitize_filename_for_chapter("", 3)
+        assert result == "03_chapter_03"
+
+    def test_index_prefix_zero_padded(self):
+        result = sanitize_filename_for_chapter("Test", 10)
+        assert result.startswith("10_")
+
+    def test_long_title_truncated_at_word_boundary(self):
+        long_title = "a" * 50 + "_" + "b" * 50
+        result = sanitize_filename_for_chapter(long_title, 1, max_len=60)
+        # Should truncate at word boundary
+        suffix = result[3:]  # Remove "01_"
+        assert len(suffix) <= 60
+
+    def test_custom_max_len(self):
+        result = sanitize_filename_for_chapter("Hello World", 1, max_len=5)
+        suffix = result[3:]  # Remove "01_"
+        assert len(suffix) <= 5
+
+    def test_hyphens_and_spaces_collapsed(self):
+        result = sanitize_filename_for_chapter("mid-night story", 1)
+        assert result == "01_mid_night_story"
+
+    def test_whitespace_collapsed(self):
+        result = sanitize_filename_for_chapter("hello   world", 1)
+        assert "hello_world" in result
+
+    def test_leading_trailing_underscores_stripped(self):
+        result = sanitize_filename_for_chapter("  hello  ", 1)
+        assert result == "01_hello"