Start runs from the dashboard: prompt textarea + POST /api/start

[suleimansh]

Part of #309. The MVP DX from #331: run $ framework, open the dashboard, type a prompt in a textarea.

• Prompt textarea + Start button on the daemon dashboard page
• POST /api/start on the daemon: spawns framework "<prompt>" as a detached child (same spawn pattern the daemon itself uses). The live view is free since the daemon already tails .framework/events.jsonl
• One-run-at-a-time guard so Start can't stack runs (the Retrieve usage limits #322 runaway concern)

Depends on #344 so the started run's gates and Stop are interactive from the daemon page. Once this exists, the #331 preset buttons are just Start with a preset prompt.