refactor(dashboard,desktop,engine): remove unused imports, props, and locals

Clears the remaining no-unused-vars warnings across the dashboard app and
server, desktop main, and engine sources. Dead React state destructures are
collapsed to setter-only, unused props are underscore-prefixed to preserve
API shape, and unreferenced catch bindings are dropped. No behaviour change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/src/chat.ts

@@ -23,8 +23,6 @@ import type {
 import { summarizeTitle } from "@fusion/core";
 import { EventEmitter } from "node:events";
 import { join, resolve, relative } from "node:path";
-import { readFile } from "node:fs/promises";
-
 import { SessionEventBuffer } from "./sse-buffer.js";
 
 // Dynamic import for @fusion/engine to avoid resolution issues in test environment