chore(frontend): remove dead code (monaco chunk, setSelection, isNew branch)

- vite.config.ts: drop the manualChunks 'monaco' branch — monaco was removed
  in the monaco→tiptap migration (82b82c5), so the regex never matches.
- use-file-manager-selection.ts: remove `setSelection` — no caller destructures
  it (file-manager.tsx never did); all writes go through rawSelectedPaths.
- settings-prompt.tsx: remove the unreachable `isNew` (promptId === 'new')
  branch — nothing routes to /settings/prompts/new, and it would render a
  "Create Prompt" header over a "Prompt not found" card. The real create path
  is `isUpdate === false`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/shared/file-manager/use-file-manager-selection.ts

@@ -39,7 +39,6 @@ interface UseFileManagerSelection {
      */
     onToggleSelection: (path: string, subtreePaths?: readonly string[]) => void;
     selectedPaths: Set<string>;
-    setSelection: (paths: Set<string>) => void;
     /** Toggle "select all": clears if everything was selected, otherwise picks every file. */
     toggleSelectAll: () => void;
 }
@@ -163,10 +162,6 @@ export function useFileManagerSelection({
         setRawSelectedPaths((prev) => (prev.size === 0 ? prev : new Set()));
     }, []);
 
-    const setSelection = useCallback((paths: Set<string>) => {
-        setRawSelectedPaths(paths);
-    }, []);
-
     return {
         clearSelection,
         isAllSelected,
@@ -174,7 +169,6 @@ export function useFileManagerSelection({
         onRowClick,
         onToggleSelection,
         selectedPaths,
-        setSelection,
         toggleSelectAll,
     };
 }

frontend/src/pages/settings/settings-prompt.tsx

@@ -640,7 +640,6 @@ function SettingsPrompt() {
         onSave: onSaveAndLeave,
     });
 
-    const isNew = promptId === 'new';
     const hasOverride =
         (activeTab === 'system' && !!promptInfo?.userSystemPrompt) ||
         (activeTab === 'human' && !!promptInfo?.userHumanPrompt);
@@ -648,9 +647,7 @@ function SettingsPrompt() {
     const pageHeader = (
         <AppHeader>
             <AppHeaderContent>
-                <AppHeaderTitle icon={<FileText className="size-4 shrink-0" />}>
-                    {isNew ? 'Create Prompt' : 'Edit Prompt'}
-                </AppHeaderTitle>
+                <AppHeaderTitle icon={<FileText className="size-4 shrink-0" />}>Edit Prompt</AppHeaderTitle>
             </AppHeaderContent>
             {promptInfo && (
                 <AppHeaderActions>
@@ -675,7 +672,7 @@ function SettingsPrompt() {
                         size="sm"
                         variant="secondary"
                     >
-                        {isNew ? 'Create' : 'Save'}
+                        Save
                     </FormSubmitButton>
                     <DropdownMenu>
                         <DropdownMenuTrigger asChild>
@@ -782,7 +779,7 @@ function SettingsPrompt() {
     const promptMeta = (
         <>
             <div className="flex flex-col gap-2 text-center">
-                <h2 className="text-2xl font-semibold">{isNew ? 'Create a prompt' : 'Edit prompt'}</h2>
+                <h2 className="text-2xl font-semibold">Edit prompt</h2>
                 <p className="text-muted-foreground">
                     {promptInfo.type === 'agent'
                         ? 'Customize the templates this agent uses'

frontend/vite.config.ts

@@ -83,9 +83,6 @@ export default defineConfig(({ mode }) => {
                         if (/[\\/]@react-pdf[\\/]/.test(id)) {
                             return 'pdf';
                         }
-                        if (/[\\/](monaco-editor|@monaco-editor[\\/]react)[\\/]/.test(id)) {
-                            return 'monaco';
-                        }
                         if (/[\\/](@tiptap|prosemirror-[a-z-]+)[\\/]/.test(id)) {
                             return 'tiptap';
                         }