fix(skills): prevent duplicate imports when import button is double-clicked (#2211)

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.

src/components/skills/UnifiedSkillsPanel.tsx

@@ -454,6 +454,7 @@ const UnifiedSkillsPanel = React.forwardRef<
       {importDialogOpen && unmanagedSkills && (
         <ImportSkillsDialog
           skills={unmanagedSkills}
+          isImporting={importMutation.isPending}
           onImport={handleImport}
           onClose={() => setImportDialogOpen(false)}
         />
@@ -600,6 +601,7 @@ interface ImportSkillsDialogProps {
     foundIn: string[];
     path: string;
   }>;
+  isImporting: boolean;
   onImport: (imports: ImportSkillSelection[]) => void;
   onClose: () => void;
 }
@@ -724,6 +726,7 @@ const RestoreSkillsDialog: React.FC<RestoreSkillsDialogProps> = ({
 
 const ImportSkillsDialog: React.FC<ImportSkillsDialogProps> = ({
   skills,
+  isImporting,
   onImport,
   onClose,
 }) => {
@@ -846,10 +849,13 @@ const ImportSkillsDialog: React.FC<ImportSkillsDialogProps> = ({
           </div>
 
           <div className="flex justify-end gap-3">
-            <Button variant="outline" onClick={onClose}>
+            <Button variant="outline" onClick={onClose} disabled={isImporting}>
               {t("common.cancel")}
             </Button>
-            <Button onClick={handleImport} disabled={selected.size === 0}>
+            <Button
+              onClick={handleImport}
+              disabled={selected.size === 0 || isImporting}
+            >
               {t("skills.importSelected", { count: selected.size })}
             </Button>
           </div>