feat(org): prevent removal of last organization owner

Add validation in OrganizationService to check if the member being
removed is the last owner of an organization. If so, raise a ValueError
to prevent accidental loss of ownership. The API layer catches this
exception and returns a 403 error with appropriate message.

src/pages/org/MembersPage.tsx

@@ -876,6 +876,30 @@ export default function MembersPage() {
         </TabsContent>
       </Tabs>
 
+      {/* ── Remove member confirmation dialog ───────────────────────────────────── */}
+      <Dialog open={removeMember !== null} onOpenChange={(open) => !open && setRemoveMember(null)}>
+        <DialogContent>
+          <DialogHeader>
+            <DialogTitle className="flex items-center gap-2">
+              <AlertTriangle className="w-5 h-5 text-destructive" />
+              Remove member from organization?
+            </DialogTitle>
+            <DialogDescription>
+              This will remove {removeMember?.user?.full_name || removeMember?.user?.email} from the organization. This action cannot be undone.
+            </DialogDescription>
+          </DialogHeader>
+          <DialogFooter>
+            <Button variant="outline" onClick={() => setRemoveMember(null)} disabled={isRemoving}>
+              Cancel
+            </Button>
+            <Button variant="destructive" onClick={handleRemoveMember} disabled={isRemoving}>
+              {isRemoving && <Loader2 className="w-4 h-4 mr-2 animate-spin" />}
+              Remove member
+            </Button>
+          </DialogFooter>
+        </DialogContent>
+      </Dialog>
+
       {/* ── User detail drawer ──────────────────────────────────────────────────── */}
     </div>
   );