Require explicit selection for initial container assignment

[frankii91]

Summary

• send only tokens already linked to the smartphone container during normal synchronization
• treat every eligible unlinked token, including tokens with a serial, as an explicit initial-assignment candidate
• send only the candidates selected in the existing assignment dialog
• persist completion of the first successful synchronization so later synchronizations cannot repeat initial assignment
• keep initial assignment pending after a canceled or failed synchronization

Root cause

templatesForAssignment was populated automatically with every eligible unlinked serial-bearing token. This happened outside the user-selection dialog, and the existing initSynced field was never set after a successful synchronization. Consequently, unrelated token details could be included during normal synchronization and initial assignment could be considered repeatedly.

Behavior after this change

• policy disabled or initial synchronization already completed: only tokens linked to this container are sent
• initial assignment active: all eligible unlinked tokens are shown for selection and only selected tokens are sent
• dialog canceled or synchronization failed: initSynced remains false and synchronization can be retried
• synchronization succeeded: initSynced is persisted as true

Validation

• flutter test --no-pub test/unit_test/api/privacy_idea_container_api_test.dart
• flutter test --no-pub test/unit_test/state_notifiers/token_container_notifier_test.dart
• both focused suites passed
• focused flutter analyze --no-pub found no errors or warnings; four pre-existing prefer_initializing_formals info diagnostics remain