Harden server startup and live-mode WebSocket behavior

[Popidge]

Motivation

• Improve reliability when starting the tester server in environments where the requested port may be taken.
• Reduce chances of silent failures or crashes from malformed requests and WebSocket errors.
• Make the Vite plugin and frontend live-mode behavior more robust and predictable across non-default ports and developer workflows.

Description

• Replace brittle server.listen(...callback...) handling with event-driven listening/error logic and a configurable maxPort retry range by introducing listenOnPort and normalizing numeric port parsing.
• Add lightweight API hardening by limiting JSON body size (express.json({ limit: '100kb')), validating POST /api/share payloads, and exposing a GET /api/config endpoint with runtime metadata.
• Improve WebSocket robustness by installing a top-level wss.on('error') handler and safer broadcast logic to avoid unhandled crashes.
• Update the Vite plugin to only run in serve mode, set Cache-Control: no-store on the injected shim to avoid stale config, and ensure the tester server is stopped when Vite shuts down.
• Revamp frontend live-mode logic to try same-origin WebSocket first (supports non-default/fallback ports), fall back to legacy :3001, deduplicate URLs, and add explicit reconnect control via shouldReconnectRef with startLiveMode/disconnectWebSocket helpers.

Testing

• Built the UI with npm run build, which completed successfully.
• Performed a server lifecycle smoke test by starting and stopping the server programmatically with a one-liner script that created new WebShareTesterServer({ port: 3101, maxPort: 3103 }), which started and stopped successfully.
• Verified port-collision fallback by binding a blocker to 3201 and starting the server with { port: 3201, maxPort: 3203 }, which logged the port busy message and successfully fell back to 3202.

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Codex Task

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