feat(relay): support configurable frps webui port and fix node detail integration

- Implement configurable FRPS WebUI switch and custom port setting (relay_frps_web_ui_port) in system configs.
- Integrate settings into Relay Node detail manage page instead of global settings.
- Dynamically query server version to select matching Docker image tag for Relay installation.
- Clean up legacy code and fix backend linter/test warnings.

internal/db/migrator/goose/sqlite/202606220003_add_frps_web_ui_config.sql

@@ -0,0 +1,9 @@
+-- +goose Up
+INSERT INTO w_system_configs (key, value, type, visibility, description, created_at, updated_at)
+VALUES 
+  ('relay_frps_web_ui_enabled', 'false', 'business', 0, '是否启用 FRPS 内置 Web 管理界面', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP),
+  ('relay_frps_web_ui_port', '17500', 'business', 0, 'FRPS 内置 Web 管理界面端口', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)
+ON CONFLICT (key) DO NOTHING;
+
+-- +goose Down
+DELETE FROM w_system_configs WHERE key IN ('relay_frps_web_ui_enabled', 'relay_frps_web_ui_port');