refactor(build_epub): replace fragile dataclass class-attr access with module constant

Language metadata (subdir, filename, title, subtitle) was stored as
EPUBConfig instance fields and then accessed as class-level attributes
in lang_map — a fragile pattern that silently breaks if any field loses
its default. Move this data to a module-level _LANG_METADATA dict and
remove the now-redundant vi_/en_/zh_ title/subtitle fields from EPUBConfig.

scripts/build_epub.py

@@ -92,6 +92,29 @@ class CoverGenerationError(EPUBBuildError):
 # Configuration and State
 # =============================================================================
 
+# Language metadata: maps lang code → (subdir, output filename, title, subtitle).
+# "" subdir means use the repo root (English content lives at the top level).
+_LANG_METADATA: dict[str, tuple[str, str, str, str]] = {
+    "en": (
+        "",
+        "claude-howto-guide.epub",
+        "Claude Code How-To Guide",
+        "Master Claude Code in a Weekend",
+    ),
+    "vi": (
+        "vi",
+        "claude-howto-guide-vi.epub",
+        "Hướng Dẫn Claude Code",
+        "Làm chủ Claude Code trong một cuối tuần",
+    ),
+    "zh": (
+        "zh",
+        "claude-howto-guide-zh.epub",
+        "Claude Code 使用指南",
+        "一个周末掌握 Claude Code",
+    ),
+}
+
 
 @dataclass
 class EPUBConfig:
@@ -109,14 +132,6 @@ class EPUBConfig:
     language: str = "en"
     author: str = "Claude Code Community"
 
-    # Language-specific metadata
-    vi_title: str = "Hướng Dẫn Claude Code"
-    vi_subtitle: str = "Làm chủ Claude Code trong một cuối tuần"
-    en_title: str = "Claude Code How-To Guide"
-    en_subtitle: str = "Master Claude Code in a Weekend"
-    zh_title: str = "Claude Code 使用指南"
-    zh_subtitle: str = "一个周末掌握 Claude Code"
-
     # Cover Settings
     cover_width: int = 600
     cover_height: int = 900
@@ -1081,29 +1096,9 @@ def main() -> int:
     repo_root = args.root if args.root else Path(__file__).parent.parent
     repo_root = repo_root.resolve()
 
-    # Set language-specific paths and metadata.
-    # Each entry: (source root, default output filename, title, subtitle)
-    lang_map: dict[str, tuple[Path, str, str, str]] = {
-        "en": (
-            repo_root,
-            "claude-howto-guide.epub",
-            EPUBConfig.en_title,
-            EPUBConfig.en_subtitle,
-        ),
-        "vi": (
-            repo_root / "vi",
-            "claude-howto-guide-vi.epub",
-            EPUBConfig.vi_title,
-            EPUBConfig.vi_subtitle,
-        ),
-        "zh": (
-            repo_root / "zh",
-            "claude-howto-guide-zh.epub",
-            EPUBConfig.zh_title,
-            EPUBConfig.zh_subtitle,
-        ),
-    }
-    root, default_output_name, title, subtitle = lang_map[args.lang]
+    # Resolve language-specific paths and metadata from the module-level constant.
+    subdir, default_output_name, title, subtitle = _LANG_METADATA[args.lang]
+    root = repo_root / subdir if subdir else repo_root
     output = args.output or (repo_root / default_output_name)
     language = args.lang