Dedicated coach chat with multiple conversations

What changed

Coach chat is now a full ChatGPT-style surface at Coach → Chat (/coach/chat): a grouped sidebar, message thread, and coach dossier. You can run multiple conversations, including several with the same coach — each thread stays tied to one coach so context does not bleed across chats.

The floating bubble still works for quick asks; it opens the same engine and can deep-link into a thread on the full page.

What you'll notice

• New conversation and Switch coach in the thread header; threads auto-title from your first message.
• Sidebar groups threads by coach (pin, archive, search).
• Per-coach theming and the dossier column on desktop; on mobile, list → thread with the dossier in a sheet.
• Browse history without a subscription; sending still respects the existing AI chat premium gate.

What didn't change

Workout runner, voice sessions, billing, and billing webhooks are untouched. Legacy chat history was migrated into threads (coach on old messages is best-effort from your current coach at migration time).