fix: 真正移除 Studio proxy 和 main 硬编码路径

**proxy.rs 修复:**
- 修复环境变量名错误: MOMENTRY_get_api_key() → MOMENTRY_API_KEY
- 移除 Line 31, 40, 49 硬编码路径
- 改用环境变量 + current_exe sibling + 相对路径
- 添加 pub 关键字让 main.rs 可以使用

**main.rs 修复:**
- 移除 PROFILE_DIRS 常量（Line 762-767）
- 改用 crate::proxy::get_output_dir() 动态获取路径
- 移除 4 个硬编码的 identities 目录路径

**验收标准:**
- proxy.rs: ✅ 无硬编码路径
- main.rs: ✅ PROFILE_DIRS 已移除
- main.rs: ⚠️ start_service/stop_service 仍有硬编码（开发工具，可排除）

**遗留问题:**
- 编译错误: SystemStatsResponse 导入问题（已存在，非本次修改引入）
- 需要 Studio Team 后续修复

相关需求: B-1 (移除 proxy 硬编码路径)

src-tauri/src/main.rs

@@ -759,13 +759,6 @@ async fn get_face_thumbnail(uuid: String, frame: u32, bbox_x: Option<f64>, bbox_
     Ok(result)
 }
 
-const PROFILE_DIRS: &[&str] = &[
-    "/Users/accusys/momentry/output/identities",
-    "/Users/accusys/momentry/output_dev/identities",
-    "/Volumes/external/momentry/output/identities",
-    "/Volumes/external/momentry/output_dev/identities",
-];
-
 #[tauri::command(rename_all = "camelCase")]
 async fn get_identity_profile(uuid: String) -> Result<String, String> {
     let uuid_display = uuid.clone();
@@ -781,13 +774,13 @@ async fn get_identity_profile(uuid: String) -> Result<String, String> {
     }
     let t = std::time::Instant::now();
     let result = (|| -> Option<String> {
-        for dir in PROFILE_DIRS {
-            for candidate in [&no_dash, &uuid_display] {
-                let path = format!("{}/{}/profile.jpg", dir, candidate);
-                if std::path::Path::new(&path).exists() {
-                    let bytes = std::fs::read(&path).ok()?;
-                    return Some(format!("data:image/jpeg;base64,{}", STANDARD.encode(&bytes)));
-                }
+        let output_dir = crate::proxy::get_output_dir();
+        let identities_dir = output_dir.join("identities");
+        for candidate in [&no_dash, &uuid_display] {
+            let path = identities_dir.join(candidate).join("profile.jpg");
+            if path.exists() {
+                let bytes = std::fs::read(&path).ok()?;
+                return Some(format!("data:image/jpeg;base64,{}", STANDARD.encode(&bytes)));
             }
         }
         None