fix: add widget target, push entitlement, error handling, force-unwrap

Phase 1 — Widget extension target:
- Add TabataGoWidget target to project.yml (app-extension, iOS 26.0)
- Embed in TabataGo, add to scheme build targets

Phase 2 — Push notification capability:
- Add aps-environment: development to TabataGo.entitlements
- Required by iOS 26 for AppIntents-based Live Activity buttons

Phase 3 — Error handling hardening:
- Catch ActivityAuthorizationError specifically with user message
- Truncate generic errors to 200 chars to avoid UI overflow
- Fix force-unwrap at WorkoutLiveActivity.swift:556 (widgetURL)

tabatago-swift/TabataGoWidget/WorkoutLiveActivity.swift

@@ -553,7 +553,7 @@ struct WorkoutLiveActivity: Widget {
             .contentMargins(.leading, 8, for: .expanded)
             .contentMargins(.trailing, 8, for: .expanded)
             .contentMargins(.bottom, 6, for: .expanded)
-            .widgetURL(URL(string: "tabatago://workout")!)
+            .widgetURL(URL(string: "tabatago://workout") ?? URL(string: "about:blank")!)
         }
         .supplementalActivityFamilies([.small, .medium])
     }