refactor: move calculate_text_length to domain, add build_chapter_payload to app-layer

- Create domain/text_utils.py with canonical calculate_text_length
  (strips chapter markers, voice markers, metadata tags)
- Remove calculate_text_length from utils.py and subtitle_utils.py
- Update all imports to use domain.text_utils directly
- Create application/chapter_selection.py with build_chapter_payload()
  (orchestrates preselection + character count + safety net)
- Replace manual chapter logic in form.py with build_chapter_payload()
- 22 new tests for text_utils and chapter_selection (1550 total)

tests/test_application_chapter_selection.py

@@ -0,0 +1,102 @@
+"""Tests for application/chapter_selection.py."""
+
+import sys
+import os
+sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), "..")))
+
+from dataclasses import dataclass
+from abogen.application.chapter_selection import build_chapter_payload
+
+
+@dataclass
+class FakeChapter:
+    title: str
+    text: str
+
+
+class TestBuildChapterPayload:
+    def test_empty_chapters(self):
+        result = build_chapter_payload([], source_name="book.txt")
+        assert len(result) == 1
+        assert result[0]["id"] == "0000"
+        assert result[0]["title"] == "book.txt"
+        assert result[0]["text"] == ""
+        assert result[0]["characters"] == 0
+        assert result[0]["enabled"] is True
+
+    def test_single_chapter_always_enabled(self):
+        chapters = [FakeChapter("Chapter 1", "Once upon a time.")]
+        result = build_chapter_payload(chapters)
+        assert len(result) == 1
+        assert result[0]["title"] == "Chapter 1"
+        assert result[0]["enabled"] is True
+        assert result[0]["index"] == 0
+        assert result[0]["id"] == "0000"
+
+    def test_content_chapters_preselected(self):
+        chapters = [
+            FakeChapter("Chapter 1", "The story begins with a long enough text to pass the threshold."),
+            FakeChapter("Chapter 2", "The story continues with another substantial chunk of text."),
+        ]
+        result = build_chapter_payload(chapters)
+        assert all(ch["enabled"] for ch in result)
+
+    def test_supplement_not_preselected(self):
+        chapters = [
+            FakeChapter("Chapter 1", "The story begins with a long enough text to pass the threshold."),
+            FakeChapter("Title Page", ""),
+            FakeChapter("Copyright", "All rights reserved."),
+            FakeChapter("Table of Contents", ""),
+            FakeChapter("Chapter 2", "The story continues with another substantial chunk of text."),
+        ]
+        result = build_chapter_payload(chapters)
+        titles_enabled = {ch["title"]: ch["enabled"] for ch in result}
+        assert titles_enabled["Chapter 1"] is True
+        assert titles_enabled["Chapter 2"] is True
+        assert titles_enabled["Title Page"] is False
+        assert titles_enabled["Copyright"] is False
+        assert titles_enabled["Table of Contents"] is False
+
+    def test_at_least_one_enabled(self):
+        chapters = [
+            FakeChapter("Title Page", ""),
+            FakeChapter("Copyright", "All rights reserved."),
+        ]
+        result = build_chapter_payload(chapters)
+        assert any(ch["enabled"] for ch in result)
+
+    def test_characters_calculated(self):
+        chapters = [FakeChapter("Ch1", "Hello world")]
+        result = build_chapter_payload(chapters)
+        assert result[0]["characters"] == 11
+
+    def test_ids_are_zero_padded(self):
+        chapters = [FakeChapter(f"Ch{i}", f"text {i}") for i in range(5)]
+        result = build_chapter_payload(chapters)
+        ids = [ch["id"] for ch in result]
+        assert ids == ["0000", "0001", "0002", "0003", "0004"]
+
+    def test_indices_are_sequential(self):
+        chapters = [FakeChapter(f"Ch{i}", f"text {i}") for i in range(3)]
+        result = build_chapter_payload(chapters)
+        indices = [ch["index"] for ch in result]
+        assert indices == [0, 1, 2]
+
+    def test_source_name_used_for_empty(self):
+        result = build_chapter_payload([], source_name="mybook.epub")
+        assert result[0]["title"] == "mybook.epub"
+
+    def test_default_source_name(self):
+        result = build_chapter_payload([])
+        assert result[0]["title"] == ""
+
+    def test_none_title_and_text(self):
+        class BadChapter:
+            def __init__(self):
+                self.title = None
+                self.text = None
+
+        result = build_chapter_payload([BadChapter()])
+        assert result[0]["title"] == ""
+        assert result[0]["text"] == ""
+        assert result[0]["enabled"] is True  # single chapter always enabled