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/gemini_config.rs

@@ -1,25 +1,10 @@
-use crate::config::write_text_file;
+use crate::config::{get_home_dir, write_text_file};
 use crate::error::AppError;
 use serde_json::Value;
 use std::collections::HashMap;
 use std::fs;
 use std::path::PathBuf;
 
-/// 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 Gemini 配置目录路径（支持设置覆盖）
 pub fn get_gemini_dir() -> PathBuf {
     if let Some(custom) = crate::settings::get_gemini_override_dir() {