feat(FN-1916): add plugin UI slots system for extensibility

- Add PluginSlot component for rendering plugin UI placeholders at designated dashboard locations
- Add usePluginUiSlots hook with 60-second TTL cache for slot definitions
- Add fetchPluginUiSlots API wrapper and PluginUiSlotEntry type
- Add UI slots documentation section to plugin authoring guide
- Add PluginSlot integration tests for TaskCard, Column, Header, SettingsModal, and TaskDetailModal
- Update CI Status example plugin to demonstrate uiSlots registration
- Fix JSX namespace import in PluginSlot.tsx for TypeScript compatibility

packages/dashboard/app/components/PluginSlot.tsx

@@ -1,3 +1,4 @@
+import type { JSX } from "react";
 import { ErrorBoundary } from "./ErrorBoundary";
 import { usePluginUiSlots } from "../hooks/usePluginUiSlots";