fix(iap): align PurchaseService with the live store products
The paywall scaffolding (PaywallBanner, locked sessions in the drawer, settings
upgrade button) was already in place but pointed at the wrong product id and
had a hard 'Pro=true' override that masked the real flow.
- kFullAccessProductId is now com.tekmidian.pailot.pro to match the IAPs
created today in App Store Connect (Apple ID 6779190925) and Google Play
Console (one-time product, Active).
- PurchaseService.initialize() no longer forces _isPro = true: it hydrates
from the SharedPreferences cache, and only grants Pro automatically in
kDebugMode when the platform store is unavailable (so sideloaded dev
builds still work). Release builds without a store fall back to the
cached value rather than silently unlocking everything.
- isProProvider's default is now false; main.dart still seeds it from
PurchaseService.instance.isPro on boot and keeps it synced via a listener.
- kFreeTierMessageTtl bumped 15m → 30m to match the stated free-tier window
(2 sessions / 30 minutes).
