Closes#2139
Two related defects let the installed-skills count balloon when users
tap the import confirm button multiple times — either deliberately or
because the button is still clickable while a slow import is in flight:
- The confirm button only disabled itself while `selected.size === 0`,
so it stayed clickable during a pending mutation. Each extra click
triggered another `importFromApps` mutation.
- `useImportSkillsFromApps` appended the server response to the
installed cache without deduping, so re-firing the mutation stacked
the same skills into the list again.
Disable the confirm (and cancel) buttons while the mutation is pending
— matching the `isRestoring` / `isDeleting` pattern already used by
`RestoreSkillsDialog` — and merge success payloads by
`InstalledSkill.id` so repeated results overwrite rather than
accumulate.
The merge is extracted as a pure `mergeImportedSkills` reducer to make
the behaviour unit-testable and to short-circuit on an empty payload,
returning the existing reference so React Query does not notify
subscribers about a no-op cache update.
