Integrate NTP promos into modal coordinator

[aibrahim-]

Collapse the New Tab Page's app-originated prompt systems from four to two by absorbing the Privacy Pro foreground promo and the Add Widget promo into the modal coordinator, so they now share its priority ordering and 24-hour cooldown instead of racing each other.

• ModalEvaluator gains a declared ModalTrigger (APP_RESUME default so the existing four evaluators are unchanged; NTP_RENDER is new).
• Coordinator filters evaluators by trigger: process onResume drives APP_RESUME; the new NewTabPageModalTrigger.onNewTabPageShown() drives NTP_RENDER.
• Privacy Pro (APP_RESUME) and Add Widget (NTP_RENDER) become ModalEvaluators that render via NewTabPageModalPresenter, which the visible tab's BrowserTabViewModel implements and registers while visible, reusing the existing CTA bottom-sheet path (pixels and dismiss handling unchanged).
• Eligibility for the Privacy Pro promo is extracted into a shared SubscriptionPromoModalDecider; the old checkSubscriptionPromoOnForeground path and the Add Widget branch of getHomeCta are removed, closing the double-prompt window.

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⚠️ API Proposal reminder

This PR changes the public surface of one or more -api modules, but the API Proposals field of the PR description is empty. This is a non-blocking reminder — please double-check whether a proposal is needed.

Changed -api source files:

• modal-coordinator/modal-coordinator-api/src/main/java/com/duckduckgo/modalcoordinator/api/ModalEvaluator.kt
• modal-coordinator/modal-coordinator-api/src/main/java/com/duckduckgo/modalcoordinator/api/ModalTrigger.kt
• modal-coordinator/modal-coordinator-api/src/main/java/com/duckduckgo/modalcoordinator/api/NewTabPageModalTrigger.kt

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