feat(skills): auto-backup skill files before uninstall

Create a local backup under ~/.cc-switch/skill-backups/ before removing
skill directories. The backup includes all skill files and a meta.json
with original skill metadata. Old backups are pruned to keep at most 20.
The backup path is returned to the frontend and shown in the success
toast. Bump version to 3.12.3.

src/components/skills/UnifiedSkillsPanel.tsx

@@ -80,9 +80,12 @@ const UnifiedSkillsPanel = React.forwardRef<
       message: t("skills.uninstallConfirm", { name: skill.name }),
       onConfirm: async () => {
         try {
-          await uninstallMutation.mutateAsync(skill.id);
+          const result = await uninstallMutation.mutateAsync(skill.id);
           setConfirmDialog(null);
           toast.success(t("skills.uninstallSuccess", { name: skill.name }), {
+            description: result.backupPath
+              ? t("skills.backup.location", { path: result.backupPath })
+              : undefined,
             closeButton: true,
           });
         } catch (error) {