fix: 设置默认账号为 demo/demo123，修复数据路径

问题修复:
1. ✅ 默认账号设置
   - db.rs: 默认密码改为 demo123
   - .env.template: 默认密码改为 demo123
   - install.sh: 默认密码改为 demo123

2. ✅ 数据路径动态化
   - db.rs: 使用 dirs::home_dir() 自动检测用户主目录
   - 默认路径: {HOME}/momentry_studio/data/users
   - 支持通过 MOMENTRY_DATA_DIR 环境变量自定义

验证脚本:
- scripts/verify_default_setup.sh: 验证默认账号和数据路径

目标:
- 安装后可直接使用 demo/demo123 登录
- 数据路径随用户主目录自动调整
- 适用于任何用户环境

影响:
- demo 用户: 密码固定为 demo123
- 数据路径: 自动检测 {HOME}/momentry_studio/data/users
- 安装即可使用，无需额外配置

src-tauri/src/db.rs

@@ -52,9 +52,14 @@ CREATE INDEX IF NOT EXISTS idx_video_marks_file ON video_marks(file_uuid);
 static DB_CONN: Mutex<Option<Connection>> = Mutex::new(None);
 
 fn db_path() -> PathBuf {
-    let base = std::env::var("MOMENTRY_DATA_DIR").unwrap_or_else(|_| "../data/users".to_string());
+    // Use dynamic path based on user home directory
+    let base = std::env::var("MOMENTRY_DATA_DIR").unwrap_or_else(|_| {
+        let home = dirs::home_dir()
+            .expect("Failed to get home directory");
+        format!("{}/momentry_studio/data/users", home.display())
+    });
     std::fs::create_dir_all(&base).ok();
-    PathBuf::from(base).join("demo.sqlite")
+    PathBuf::from(base).join("demo.sqlite")  // TODO: Change to dynamic username
 }
 
 pub fn init_db() -> Result<(), String> {
@@ -66,19 +71,11 @@ pub fn init_db() -> Result<(), String> {
 // Add password_hash column if not exists (migration)
     conn.execute("ALTER TABLE app_users ADD COLUMN password_hash TEXT", []).ok();
 
-    // Get default user credentials from environment or use generated values
+    // Get default user credentials from environment or use demo123 as default
     let default_id = "demo";
     let default_user = "demo";
     let default_password = std::env::var("MOMENTRY_DEFAULT_PASSWORD")
-        .unwrap_or_else(|_| {
-            // Generate a random password if not set
-            use std::time::{SystemTime, UNIX_EPOCH};
-            let timestamp = SystemTime::now()
-                .duration_since(UNIX_EPOCH)
-                .unwrap()
-                .as_secs();
-            format!("demo_{}", timestamp % 1000000)  // demo_123456
-        });
+        .unwrap_or_else(|_| "demo123".to_string());  // Default: demo123
     
     // Check if demo user exists and has password
     let password_hash: Option<String> = conn