feat(frontend): file:// bundle support (HashRouter + build:desktop)

- main.tsx: use HashRouter when loaded over file:// (packaged desktop
  has no server for History API path routing); keep BrowserRouter on
  the http web build.
- add build:desktop script (vite build --base=./ --outDir dist-desktop)
  so assets resolve under file://; gitignore dist-desktop.

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

.gitignore

@@ -22,3 +22,4 @@ dist-ssr
 *.njsproj
 *.sln
 *.sw?
+dist-desktop