feat: WebUI settings page with sidebar + ConfigRegistry integration

- /api/v1/settings endpoint: GET (list by prefix) + PUT (set key/value)
- Config tab redesigned: left sidebar (core + per-plugin) + right editor
- Sidebar groups: core.* and plugin.<name>.* from plugin registry
- Each setting rendered as editable input with save button
- WebUI now receives cfgReg and pluginReg from main
- handler_test.go updated with new NewHandler signature

cmd/homed/main.go

@@ -303,7 +303,7 @@ func main() {
 	// === Built-in HTTP API & WebUI Plugin ===
 	webui := api.NewWebUIPlugin(
 		"webui", cfg.Daemon.ListenAddr,
-		sup, memDB, skMgr, luaVM, cfg, iom, textMem, ks, trk,
+		sup, memDB, skMgr, luaVM, cfg, iom, textMem, ks, trk, cfgReg, pluginReg,
 	)
 	iom.RegisterDevice(webui)
 	webui.Start()