fix: Extend offline grace period to 90 days for Issue #15

This commit addresses the concern about the restrictive 7-day offline
grace period that prevented extended offline use.

Problem:
- OFFLINE_GRACE_PERIOD_DAYS was set to 7 days
- Users planning multi-week trips or working in remote locations couldn't use the app
- The app would stop working after just one week offline

Solution:
- Extended OFFLINE_GRACE_PERIOD_DAYS from 7 to 90 days (3 months)
- Provides sufficient time for extended travel, field work, and remote deployments
- Maintains reasonable validation requirements for license integrity

Impact:
- Licensed users can now work offline for up to 90 days
- Covers most realistic offline scenarios (vacations, expeditions, deployments)
- After 90 days, users see clear "Trial Expired" message with purchase/activation options
- Balanced approach: generous enough for real use cases, not indefinite

Testing:
- Code compiles successfully
- License tests pass
- Grace period logic verified to use this single constant

Fixes #15

Author: moinulmoin

src-tauri/src/commands/license.rs

@@ -17,7 +17,7 @@ struct CachedLicenseStatus {
 }
 
 // Constants for cache and grace periods
-const OFFLINE_GRACE_PERIOD_DAYS: i64 = 7; // 7 days offline grace for licensed users
+const OFFLINE_GRACE_PERIOD_DAYS: i64 = 90; // 90 days offline grace for licensed users - supports extended offline use
 const LICENSE_CACHE_KEY: &str = "license_status";
 const LAST_VALIDATION_KEY: &str = "last_license_validation";