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/settings.rs

@@ -163,18 +163,11 @@ impl Default for AppSettings {
 impl AppSettings {
     fn settings_path() -> Option<PathBuf> {
         // settings.json 保留用于旧版本迁移和无数据库场景
-        #[cfg(windows)]
-        if let Ok(home) = std::env::var("HOME") {
-            let trimmed = home.trim();
-            if !trimmed.is_empty() {
-                return Some(
-                    PathBuf::from(trimmed)
-                        .join(".cc-switch")
-                        .join("settings.json"),
-                );
-            }
-        }
-        dirs::home_dir().map(|h| h.join(".cc-switch").join("settings.json"))
+        Some(
+            crate::config::get_home_dir()
+                .join(".cc-switch")
+                .join("settings.json"),
+        )
     }
 
     fn normalize_paths(&mut self) {