feat(settings): unified settings modal with context switcher (Phase 1)
New unified settings modal accessible via `,` keyboard shortcut.
Three-context switcher: Site (with site dropdown), Workspace, Account.
Horizontal tabs per context with animated transitions.
Phase 1 tabs implemented:
- Site → General (name, timezone, domain, tracking script with copy)
- Site → Goals (CRUD with inline create/edit)
- Workspace → General (org name, slug, danger zone)
- Workspace → Billing (plan card, usage, cancel/resume, portal)
- Account → Profile (wraps existing ProfileSettings)
Phase 2 tabs show "Coming soon" placeholder:
- Site: Visibility, Privacy, Bot & Spam, Reports, Integrations
- Workspace: Members, Notifications, Audit Log
- Account: Security, Devices
Old settings pages and profile modal remain functional.
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