feat: add environment labels (local/dev/stage/prod) for connections

Visual badges help distinguish environments across the UI to prevent
running dangerous queries on production. Environment color takes
priority over custom connection color for the toolbar border.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/src/models/connection.rs

@@ -11,6 +11,7 @@ pub struct ConnectionConfig {
     pub database: String,
     pub ssl_mode: Option<String>,
     pub color: Option<String>,
+    pub environment: Option<String>,
 }
 
 impl ConnectionConfig {