fix: restore workspaces after cold restart

[web3dev1337]

Summary

• only offer session recovery when the current server boot has not already rehydrated that workspace's sessions
• restore the recovery prompt for auto-open and reconnect flows, not just dashboard workspace opens
• centralize dashboard recovery planning so manual open and automatic workspace entry use the same decision path

Testing

• node --check server/sessionManager.js
• node --check server/index.js
• node --check client/dashboard.js
• node --check client/app.js
• npm run lint (fails: missing script in package.json)