fix(files): hide desktop-only reveal action and label download in browser clients

Reveal-in-file-manager was always offered whenever the server exposed
revealPath, including in a plain browser tab where there is no local
file manager to reveal into. Gate it behind a new isBrowserClientRuntime
check (web platform, no Electron shell) and relabel the save action to
"Download" for that case, since it triggers a browser-style file
download rather than an in-place save.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packages/ui/src/lib/desktop.ts

@@ -1,4 +1,4 @@
-import type { ProjectEntry, TerminalShell } from '@/lib/api/types';
+import type { ProjectEntry, RuntimeAPIs, TerminalShell } from '@/lib/api/types';
 import { getInjectedBootOutcome } from '@/lib/desktopBoot';
 import type { DraftStarterRef } from '@/lib/draftStarters';
 import type { MobileKeyboardMode } from '@/lib/mobileKeyboardMode';
@@ -562,6 +562,15 @@ export const isWebRuntime = (): boolean => {
   return !isVSCodeRuntime();
 };
 
+/**
+ * Electron reuses the web RuntimeAPIs implementation, so distinguish a browser
+ * client from an Electron renderer with both the runtime descriptor and shell.
+ */
+export const isBrowserClientRuntime = (
+  platform: RuntimeAPIs['runtime']['platform'],
+  desktopShell = isDesktopShell(),
+): boolean => platform === 'web' && !desktopShell;
+
 export const getDesktopHomeDirectory = async (): Promise<string | null> => {
   if (typeof window !== 'undefined') {
     const embedded = window.__OPENCHAMBER_HOME__;