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>

package.json

@@ -7,7 +7,8 @@
     "dev": "vite",
     "build": "tsc -b && vite build",
     "lint": "eslint .",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "build:desktop": "tsc -b && vite build --base=./ --outDir dist-desktop"
   },
   "dependencies": {
     "axios": "^1.16.0",