feat(iap): add freemium unlock via one-time in-app purchase

Adds a permanent, non-consumable premium unlock (EstadoEntitlement +
PuertoCompras/ServicioComprasPlayBilling) that removes ads and unlocks
alarm vacations, alarms past a 5-alarm free cap, recording start, and
full Android Auto browsing. The phone equalizer stays free for everyone.

- Entitlement is prefs-backed (compra_premium_v1), fail-open, and
  resolvable headlessly via esPremiumPersistido() for the Android Auto
  audio handler, which registers before runApp.
- Android Auto reduced mode keeps the real root folder labels for free
  users; browsing into any of them (and playFromMediaId/playFromSearch/
  skipToNext/skipToPrevious) is blocked at the getChildren/servicio_audio
  choke points, with a locked "Función Premium" item as the backstop.
  Current-station play/pause/stop stays untouched. A free -> premium
  transition actively invalidates the head unit's cached browse tree.
- Ads (top banner + capped interstitial before adding a station or an
  alarm) are gated behind entitlement via ServicioAnuncios, using
  official Google test ad unit IDs pending AdMob provisioning.
- Alarm cap UX shows an explanatory message with a secondary unlock
  action rather than a bare paywall jump; existing data is grandfathered.
- 4 new localization keys translated across all 13 supported locales.

Co-located tests use strict TDD (RED test before implementation) for
every new pure-logic unit; full existing suite passes unchanged.

openspec/changes/iap-freemium-unlock/specs/premium-entitlement/spec.md

@@ -0,0 +1,73 @@
+# Premium Entitlement Specification
+
+## Purpose
+
+Track whether the current user holds the permanent, non-consumable premium unlock, and expose that flag to every gated surface (freemium-gating, ad-display, android-auto-media) both from the UI layer and headlessly (Android Auto, registered before `runApp`).
+
+## Requirements
+
+### Requirement: One-Time Non-Consumable Purchase
+
+The system MUST let the user buy a single non-consumable product via `in_app_purchase` from the Settings row or any contextual upsell. On a successful purchase, entitlement MUST flip to premium immediately, app-wide, with no restart required.
+
+#### Scenario: Successful purchase
+- GIVEN a free-tier user taps "buy premium" from Settings or a contextual upsell
+- WHEN the purchase completes successfully
+- THEN entitlement becomes premium immediately, without restarting the app
+
+#### Scenario: Purchase cancelled or failed
+- GIVEN a free-tier user starts the purchase flow
+- WHEN the user cancels or the purchase fails
+- THEN entitlement remains free tier, and no charge or partial state is left behind
+
+#### Scenario: Already-purchased attempt is idempotent
+- GIVEN a user already holds premium entitlement
+- WHEN they somehow re-trigger the buy flow
+- THEN no duplicate charge occurs and entitlement stays premium
+
+### Requirement: Restore Purchases
+
+Settings MUST expose a "restore purchases" action that re-queries Play Billing and unlocks entitlement when a prior purchase is found.
+
+#### Scenario: Restore finds a prior purchase
+- GIVEN a reinstall or new device with no local entitlement flag
+- WHEN the user taps "restore purchases" and a valid purchase exists on the Play account
+- THEN entitlement becomes premium
+
+#### Scenario: Restore finds nothing
+- GIVEN a user with no prior purchase
+- WHEN they tap "restore purchases"
+- THEN the user stays on the free tier with a clear, non-error-looking result (not a crash or ambiguous failure)
+
+### Requirement: Persisted, Fail-Open Entitlement
+
+Entitlement MUST persist locally under a versioned key (e.g. `compra_premium_v1`) and MUST be readable offline. If an entitlement check cannot complete (no network, Play Billing unreachable), the system MUST fail-open: trust the last persisted flag rather than lock out a payer.
+(Previously: no entitlement concept existed.)
+
+#### Scenario: Offline cold start after purchase
+- GIVEN a user purchased premium previously
+- WHEN they open the app fully offline
+- THEN premium entitlement is honored from the persisted flag
+
+#### Scenario: Failed check does not falsely grant premium
+- GIVEN a free-tier user with no persisted premium flag
+- WHEN an entitlement check fails
+- THEN the user remains free tier (fail-open trusts the last flag, it does not invent one)
+
+### Requirement: Headless-Safe Entitlement Read
+
+Entitlement MUST be resolvable via a prefs-lazy fallback (no `BuildContext`/`Provider` dependency), for callers such as `PluriWaveAudioHandler` that register before the widget tree exists.
+
+#### Scenario: Android Auto cold start
+- GIVEN the audio handler is constructed before `runApp`
+- WHEN it needs to know the current entitlement to build the browse tree
+- THEN it resolves entitlement via the prefs-lazy path without requiring a `Provider`
+
+### Requirement: Instant Unlock Propagation
+
+A successful purchase or restore MUST notify all listeners (top banner, gated screens, cached Android Auto entitlement) immediately, without an app restart.
+
+#### Scenario: Banner disappears immediately on purchase
+- GIVEN the ad banner is visible when the user completes a purchase
+- WHEN the purchase confirms
+- THEN the banner disappears immediately, with no restart