chore(documents): remove legacy /documents/files route + folder tree

The /documents/files page rendered a storagePath-prefix folder tree
disconnected from document_folders. Replaced by the unified hub
(Task 15). 301 redirect catches stray bookmarks. file-browser-store
repurposed to hold the document_folders.id selection.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/stores/file-browser-store.ts

@@ -1,26 +1,15 @@
 import { create } from 'zustand';
 
-interface FileBrowserStore {
+interface FileBrowserState {
   viewMode: 'grid' | 'list';
-  currentFolder: string;
-  selectedFiles: string[];
   setViewMode: (mode: 'grid' | 'list') => void;
-  setCurrentFolder: (folder: string) => void;
-  toggleFileSelection: (fileId: string) => void;
-  clearSelection: () => void;
+  selectedFolderId: string | null | undefined; // undefined = no selection, null = root
+  setSelectedFolderId: (id: string | null | undefined) => void;
 }
 
-export const useFileBrowserStore = create<FileBrowserStore>((set) => ({
-  viewMode: 'grid',
-  currentFolder: '',
-  selectedFiles: [],
-  setViewMode: (mode) => set({ viewMode: mode }),
-  setCurrentFolder: (folder) => set({ currentFolder: folder, selectedFiles: [] }),
-  toggleFileSelection: (fileId) =>
-    set((state) => ({
-      selectedFiles: state.selectedFiles.includes(fileId)
-        ? state.selectedFiles.filter((id) => id !== fileId)
-        : [...state.selectedFiles, fileId],
-    })),
-  clearSelection: () => set({ selectedFiles: [] }),
+export const useFileBrowserStore = create<FileBrowserState>((set) => ({
+  viewMode: 'list',
+  setViewMode: (viewMode) => set({ viewMode }),
+  selectedFolderId: undefined,
+  setSelectedFolderId: (selectedFolderId) => set({ selectedFolderId }),
 }));