feat(mcp): show inline duplicate ID error and block submit in add mode

- Display red hint next to title when ID exists

- Disable Add/Save button and prevent submit on duplicate

- Accept existing IDs via prop for real-time validation

- Remove overwrite confirmation dialog on add

- i18n: add duplicate-ID error strings and remove unused overwrite prompt

- files:

  - src/components/mcp/McpFormModal.tsx

  - src/components/mcp/McpPanel.tsx

  - src/i18n/locales/en.json

  - src/i18n/locales/zh.json

src/components/mcp/McpPanel.tsx

@@ -292,6 +292,7 @@ const McpPanel: React.FC<McpPanelProps> = ({ onClose, onNotify }) => {
         <McpFormModal
           editingId={editingId || undefined}
           initialData={editingId ? servers[editingId] : undefined}
+          existingIds={Object.keys(servers)}
           onSave={handleSave}
           onClose={handleCloseForm}
         />