refactor(prd-editor): modularize PRD editor with typed feature modules

Break the legacy PRDEditor.jsx monolith into a feature-based TypeScript architecture under src/components/prd-editor while keeping behavior parity and readability.

Key changes:

- Replace PRDEditor.jsx with a typed orchestrator component and a compatibility export bridge at src/components/PRDEditor.tsx.

- Split responsibilities into dedicated hooks: document loading/init, existing PRD registry fetching, save workflow with overwrite detection, and keyboard shortcuts.

- Split UI into focused view components: header, editor/preview body, footer stats, loading state, generate-tasks modal, and overwrite-confirm modal.

- Move filename concerns into utility helpers (sanitize, extension handling, default naming) and centralize template/constants.

- Keep component-local state close to the UI that owns it (workspace controls/modal toggles), while shared workflow state remains in the feature container.

- Reuse the existing MarkdownPreview component for safer markdown rendering instead of ad-hoc HTML conversion.

- Update TaskMasterPanel integration to consume typed PRDEditor directly (remove any-cast) and pass isExisting metadata for correct overwrite behavior.

- Keep all new/changed files below 300 lines and add targeted comments where behavior needs clarification.

Validation:

- npm run typecheck

- npm run build

src/components/prd-editor/hooks/usePrdRegistry.ts

@@ -0,0 +1,50 @@
+import { useCallback, useEffect, useState } from 'react';
+import { api } from '../../../utils/api';
+import type { ExistingPrdFile, PrdListResponse } from '../types';
+
+type UsePrdRegistryArgs = {
+  projectName?: string;
+};
+
+type UsePrdRegistryResult = {
+  existingPrds: ExistingPrdFile[];
+  refreshExistingPrds: () => Promise<void>;
+};
+
+function getPrdFiles(data: PrdListResponse): ExistingPrdFile[] {
+  return data.prdFiles || data.prds || [];
+}
+
+export function usePrdRegistry({ projectName }: UsePrdRegistryArgs): UsePrdRegistryResult {
+  const [existingPrds, setExistingPrds] = useState<ExistingPrdFile[]>([]);
+
+  const refreshExistingPrds = useCallback(async () => {
+    if (!projectName) {
+      setExistingPrds([]);
+      return;
+    }
+
+    try {
+      const response = await api.get(`/taskmaster/prd/${encodeURIComponent(projectName)}`);
+      if (!response.ok) {
+        setExistingPrds([]);
+        return;
+      }
+
+      const data = (await response.json()) as PrdListResponse;
+      setExistingPrds(getPrdFiles(data));
+    } catch (error) {
+      console.error('Failed to fetch existing PRDs:', error);
+      setExistingPrds([]);
+    }
+  }, [projectName]);
+
+  useEffect(() => {
+    void refreshExistingPrds();
+  }, [refreshExistingPrds]);
+
+  return {
+    existingPrds,
+    refreshExistingPrds,
+  };
+}