fix: stop a half-completed media collection move reporting success (#6017)

[atomantic]

Summary

Moving items between media collections is two writes: add to the target, then remove from the source. bulkMoveOrCopy in client/src/pages/MediaCollectionDetail.jsx swallowed every removal error into null (.catch(() => null)) and computed its failure count as selectedItems.length - added - dupes — which only accounts for add failures. A move whose removals all failed still toasted Moved N to "…" while the items sat duplicated in both collections, and exitSelectMode() cleared the selection so the user had nothing left to retry.

This change:

• Tracks the keys whose removal failed in a removeFailedKeys set, separate from the add-failure count.
• Never toasts success when a removal failed. The message is Copied N to "<target>", but M could not be removed from "<source>" — deliberately "Copied", because calling a half-completed move a "Move" is exactly the false success this branch exists to prevent. An add failure appends ; K failed to add to the same message.
• Still reconciles collection to the authoritative server state returned by the last successful removal, so a partial failure doesn't leave the view showing items that did move out.
• Keeps select mode open with only the half-moved items selected, so the move (or a plain Remove) can be re-run on exactly what failed.

The synthetic Unsorted source is unaffected — it never issues removals, it just re-derives via refresh().

Rejected alternative (per the issue): rolling back the adds in the target collection with automated DELETEs. Accurate partial-progress reporting leaves the user in control instead of cascading more writes onto an already-unstable destination.

Test plan

• client/src/pages/MediaCollectionDetail.test.jsx gains three cases covering bulkMoveOrCopy: every removal succeeds (plain success toast, select mode exits), every removal fails (error toast, no success toast, all items stay selected), and a mixed run where one add and one removal fail (both counts reported, collection reconciled to the last successful removal, selection narrowed to the one failed key). BulkTargetPicker is now mocked with a clickable destination row so the flow can be driven end to end.
• Verified the two failure-path tests fail against the pre-fix component and pass after it.
• cd client && npx vitest run — 832 files / 10220 tests pass. One unrelated pre-existing date-boundary flake in src/components/calendar/ReviewTab.test.jsx failed on the first run and passed on re-run; it is untouched by this diff.

Closes #6017

https://claude.ai/code/session_01VjkWVTfzKyRuAv3HEsspwN