feat(user-profile): gate Save on a real edit and discard it on back

[bmc08gt]

Opening Change Display Name or Change Username armed the confirm button before anything had been edited. Both steps seed the field from the stored profile, and the button was gated on the text merely being non-blank, so the first tap submitted the value that was already there.

Both view models now keep the loaded value in state as a baseline and compare the field against it. One character's difference arms the button; typing the original back in disarms it. Nodes 9553:113166 and 9553:113168.

Leaving a step throws the edit away. The back icon dispatches DiscardChanges, which puts the baseline back in the field. A gesture back lands in the same place by a different route: AppNavHost installs rememberViewModelStoreNavEntryDecorator, so each step's ViewModel is scoped to its nav entry and goes with the pop. No BackHandler — adding one would have cost predictive-back animation on these screens.

Set Profile Picture opened on an empty well with a "+", so an account that already had a picture looked like it had none. It now seeds from the stored picture and falls back to it when a pick is discarded. The seeded value is a server-side MediaItem, never a local Uri, so it cannot be mistaken for a pick — Save stays disabled until one is made. That gate moved into State.isChanged, so all three steps express their dirty check the same way.

Two more fixes came out of this:

• The name and username fields could be clobbered mid-edit. Both seeded on every profile emission, and the 60s poll publishes UserProfile.Empty when it cannot find the server profile — a different value, so distinctUntilChanged let it through and it overwrote whatever was being typed. The field now follows the store only while it is untouched; the baseline still moves either way.
• An abandoned photo pick left a re-encoded file in the cache with nothing to delete it. PhotoSelectionViewModel.onCleared now drops it, which covers the gesture back and the successful upload that makes the local copy redundant.

Adds 24 unit tests over the three reducers and their computed state.