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