refactor: simplify _build_request, restore epub3_export as config object

Simplify _build_request() in WebUI
- Remove profiles loading (load_profiles, normalize_profile_entry)
- Remove PronunciationConfig/Epub3ExportConfig building
- Pass raw fields directly to ConversionRequest
- Clean up unused imports

Bugfix: epub3_export as Optional[Epub3ExportConfig] (None = disabled)
- Reverted generate_epub3: bool + epub3_book_id: str back to object pattern
- Consistent with word_substitution, subtitle_input, chapter_chunk
- Updated conversion_service.py _finalize() to use request.epub3_export

abogen/application/conversion_request.py

@@ -16,6 +16,7 @@ from typing import Any, Dict, List, Optional
 
 from abogen.application.conversion_config import (
     ChapterChunkConfig,
+    Epub3ExportConfig,
     SubtitleInputConfig,
     WordSubstitutionConfig,
 )
@@ -106,11 +107,8 @@ class ConversionRequest:
     cover_image_path: Optional[Path] = None
     cover_image_mime: Optional[str] = None
 
-    # --- Feature toggles ---
-    generate_epub3: bool = False
-    epub3_book_id: str = ""
-
     # --- Feature configs (None = disabled) ---
+    epub3_export: Optional[Epub3ExportConfig] = None
     word_substitution: Optional[WordSubstitutionConfig] = None
     subtitle_input: Optional[SubtitleInputConfig] = None
     chapter_chunk: Optional[ChapterChunkConfig] = None

abogen/application/conversion_service.py

@@ -136,7 +136,7 @@ def _finalize(
             raise RuntimeError(f"Failed to embed m4b metadata: {exc}") from exc
 
     # EPUB3 generation
-    if request.generate_epub3 and plan.extraction:
+    if request.epub3_export and plan.extraction:
         audio_asset = result.audio_path
         if not audio_asset and result.chapter_paths:
             audio_asset = result.chapter_paths[0]
@@ -153,7 +153,7 @@ def _finalize(
                 events.log("Generating EPUB 3 package...")
                 epub_path = build_epub3_package(
                     output_path=epub_output_path,
-                    book_id=request.epub3_book_id,
+                    book_id=request.epub3_export.book_id,
                     extraction=plan.extraction,
                     metadata_tags=result.metadata or {},
                     chapter_markers=result.chapter_markers or [],
@@ -183,7 +183,7 @@ def _finalize(
             chunk_level=request.chapter_chunk.chunk_level if request.chapter_chunk else None,
             speaker_mode=request.chapter_chunk.speaker_mode if request.chapter_chunk else None,
             speakers=request.chapter_chunk.speakers if request.chapter_chunk else None,
-            generate_epub3=bool(request.generate_epub3),
+            generate_epub3=bool(request.epub3_export),
         )
 
         metadata_dir = plan.output_layout.metadata_dir