feat: TOON analytics — privacy-first build-time analytics injection

[sebastienrousseau]

Deliverables

• New AnalyticsPlugin that injects a lightweight, privacy-first analytics snippet at build time
• Support Plausible, Umami, and a custom self-hosted endpoint
• Configurable via [analytics] in ssg.toml (provider, domain, script_url)
• Respect Do Not Track and GDPR consent signals in the injected script
• No third-party cookies — ever
• Per-tenant analytics isolation when used with the TOON multi-tenant engine