Treat feed IDs as unsigned everywhere
This fixes several bugs introduced by commit 230b8f20 ("Port all instance of KRandom") which switched the random number generator used in FeedList::generateId() from KRandom to QRandomGenerator. The former generates positive signed integers while the latter generates unsigned ints. While TreeNode already used uint to store the IDs, other classes used plain int. Half of the newly generated IDs thus got mapped to negative values, confusing various validity checks.
BUG: 429444