refactor(config): consolidate get_home_dir into single public function

Follow-up to #644: Extract duplicate get_home_dir() implementations
into a single pub fn in config.rs, reducing code duplication across
codex_config.rs, gemini_config.rs, and settings.rs.

Also adds documentation comments to build.rs explaining the Windows
manifest workaround for test binaries.

src-tauri/src/codex_config.rs

@@ -2,28 +2,14 @@
 use std::path::PathBuf;
 
 use crate::config::{
-    atomic_write, delete_file, sanitize_provider_name, write_json_file, write_text_file,
+    atomic_write, delete_file, get_home_dir, sanitize_provider_name, write_json_file,
+    write_text_file,
 };
 use crate::error::AppError;
 use serde_json::Value;
 use std::fs;
 use std::path::Path;
 
-/// 获取用户主目录，带回退和日志
-fn get_home_dir() -> PathBuf {
-    #[cfg(windows)]
-    if let Ok(home) = std::env::var("HOME") {
-        let trimmed = home.trim();
-        if !trimmed.is_empty() {
-            return PathBuf::from(trimmed);
-        }
-    }
-    dirs::home_dir().unwrap_or_else(|| {
-        log::warn!("无法获取用户主目录，回退到当前目录");
-        PathBuf::from(".")
-    })
-}
-
 /// 获取 Codex 配置目录路径
 pub fn get_codex_config_dir() -> PathBuf {
     if let Some(custom) = crate::settings::get_codex_override_dir() {