Fix: Start UI event listener before spawning workers [Vibecoded]

[rappie]

WARNING: this is 100% vibecoded

Successfully tested locally.

Summary

Start the UI/log event listener before spawning campaign workers.

spawnListener uses dupChan, so it only receives events written after the listener is attached.
Previously, workers were spawned first, which meant early worker events could be missed if they were emitted before the UI/log listener started.

Why

Worker startup and early replay events are emitted immediately after worker threads start. If the listener subscribes too late, text output can miss early events such as Starting FuzzerAgent, making logs incomplete and downstream tooling harder to reason about.

Changes

• Delay worker spawning behind a local spawnWorkers action.
• Start uiEventsForwarderStopVar <- spawnListener forwardEvent before spawning workers.
• Preserve existing interactive and non-interactive behavior otherwise.