fix(desktop): relay Brain analytics with Desktop identity - #7356

After Desktop emits module_open, embedded Brain events use a different analytics context and cannot read Desktop's persisted UUID. Deployment creation can be blocked by GTM's missing-identity rule, and the current GTM whitelist omits onboarding/cancellation events.

Expose an optional analytics capability through the existing host-config API and accept Brain events through the existing SDK event bus. Event callbacks receive browser-provided source/origin metadata; the relay accepts only the running Brain iframe and its installed origin. GTM-covered events enter the parent dataLayer once, inheriting the canonical Desktop UUID and attribution. Events absent from the GTM whitelist use the parent Rybbit SDK once, identifying with that same UUID and respecting the primitive/2KB property contract. Direct delivery waits for asynchronous SDK loading; event fields are allowlisted.

Companion Brain PR: labring/brain#359. Deploy this Desktop change first for immediate continuity. Either PR remains independently mergeable; Brain negotiates capability and keeps its previous transport on older Desktop deployments. Existing website token-login tracking from main is retained.

Validation:

• 19 focused tests passed: pnpm test:ci src/__tests__/unit/utils/brain-analytics.test.ts src/__tests__/unit/utils/brain-analytics-sdk.test.ts src/__tests__/unit/utils/gtm.test.ts src/__tests__/unit/utils/marketing-attribution.test.ts from frontend/desktop.
• pnpm exec tsc --noEmit --incremental false from frontend/desktop.
• pnpm build from frontend/packages/client-sdk.
• Coverage includes real SDK sender propagation/acknowledgement, iframe/origin rejection, one-time GTM/Rybbit dispatch, UUID identity, late script loading, byte limits, and exclusion of credentials/feedback text.