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/planning.ts

@@ -26,10 +26,8 @@ import type { AiSessionStore, AiSessionRow } from "./ai-session-store.js";
 import { SessionEventBuffer, type SessionBufferedEvent } from "./sse-buffer.js";
 import {
   createSessionDiagnostics,
-  setDiagnosticsSink,
   resetDiagnosticsSink,
   nonfatal,
-  nonfatalAsync,
 } from "./ai-session-diagnostics.js";
 
 // Dynamic import for @fusion/engine to avoid resolution issues in test environment