Replace force-unwrap of colorGradient.id with explicit ValidationError

[Copilot]

toDb() and toInsertCompanion() on Pocket were force-unwrapping colorGradient.id!, causing a crash if a non-persisted or fallback gradient (id == null) was ever used.

Changes

• toDb() / toInsertCompanion(): Replace colorGradient.id! with an explicit null-check that throws ValidationError with a descriptive message

// Before
colorGradientId: colorGradient.id!,

// After
final gradientId = colorGradient.id;
if (gradientId == null) {
  throw ValidationError(
    'Cannot update pocket: the selected color gradient has not been saved to the database. '
    'Ensure a valid gradient is chosen before updating a pocket.',
    StackTrace.current,
  );
}

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