feat: add plugin loader with manifest validation and crash restart

Validates an unpacked plugin, spawns a worker_threads worker and manages
activate/deactivate/crash-restart with backoff, disabling after three
tries.

The crash counter clears only after a plugin stays up past a stability
window, not on activation. Clearing it on activation let a plugin that
activates cleanly and then dies restart forever, because every attempt
looked like the first.

loader.ts documents what the worker boundary does and does not buy you.
Node's permission model is process-wide, not per-worker, so it cannot be
enabled for plugin workers alone and is not claimed here.

src/backend/utils/logger.ts

@@ -309,5 +309,6 @@ export const versionLogger = new Logger("VERSION", "📦", "#8b5cf6");
 export const dashboardLogger = new Logger("DASHBOARD", "📊", "#ec4899");
 export const guacLogger = new Logger("GUACAMOLE", "🖼️", "#ff6b6b");
 export const homepageLogger = new Logger("HOMEPAGE", "🏠", "#f97316");
+export const pluginLogger = new Logger("PLUGIN", "🧩", "#84cc16");
 
 export const logger = systemLogger;