feat(admob): add sample preloader controls

[polmiro]

Summary

Expands the Google Mobile Ads Next-Gen example app with reusable preloader controls:

• add reusable controls for starting and stopping format preloaders
• expose buffer capacity and available-ad state
• keep buffer configuration editable only before a preloader starts

Screenshots

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Note

Low Risk
Sample-app-only UI helpers with no changes to SDK, billing, or production ad integration paths.

Overview
Adds reusable Compose UI for the AdMob Next-Gen sample so format screens can demo ad preloading without duplicating layout code.

PreloaderUi.kt introduces PreloaderUiState (backed by PreloadConfiguration and an ads-available provider), rememberPreloaderUiState, and PreloaderPanel: start/stop toggle, running/stopped badge, buffer capacity steppers (1–5, locked while running), live “ads ready” count with ~500ms polling, status messages after start/stop/preload events, plus optional extra metrics and action buttons.

AdSupport.kt adds preloadStatusCallback, a small PreloadCallback that hops preload success, failure, and buffer-exhausted events onto a CoroutineScope so the panel can update safely from SDK callbacks.

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[emerge-tools]

📸 Snapshot Test

1 modified, 620 unchanged

Name	Added	Removed	Modified	Renamed	Unchanged	Errored	Approval
TestPurchasesUIAndroidCompatibility Paparazzi com.revenuecat.testpurchasesuiandroidcompatibility.paparazzi	0	0	0	0	282	0	N/A
TestPurchasesUIAndroidCompatibility com.revenuecat.testpurchasesuiandroidcompatibility	0	0	1	0	338	0	✅ Approved

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