Add multi-provider analytics with Plausible

[thebuilder]

Summary

• Refactor the package around provider-neutral analytics contracts while preserving existing Vercel connections.
• Add Plausible Cloud reporting, shared and per-connection credentials, capabilities, events, configurable event-property discovery, and provider-aware filtering.
• Redesign the settings and analytics UI for provider selection, conditional features, event drilldown, local provider branding, stable loading states, and a focused floating save action.
• Update the README and Marketplace documentation, configuration reference, and screenshots for the provider-aware model.

Why

Web Analytics was coupled to Vercel even though connections and report shapes can support multiple vendors. This change introduces an explicit provider and capability boundary so the UI only requests features each adapter supports, and adds Plausible as the first additional provider.

Impact

• Existing serialized connections without a provider remain Vercel connections.
• Vercel behavior continues through the provider-neutral client contract.
• Plausible Cloud users can configure a site ID and Stats API key, query common traffic reports, inspect goals, and configure custom event properties.
• Provider credential readiness appears when creating connections; saved connections expose their credential source and connection-test status.
• Credentials stay in server configuration and are never returned to the backoffice.

Validation

• dotnet test TheBuilder.WebAnalytics.slnx --no-restore — 214 passed
• corepack pnpm check
• corepack pnpm test — 165 passed
• corepack pnpm build