Make app fully portable, remove installers

- Remove MSI and NSIS installer targets, only produce vesper.exe
- Remove tauri-plugin-window-state, replace with manual window state
  save/restore to data/window-state.json next to the exe
- Redirect WebView2 user data (including localStorage) to data/
  folder next to the exe via WEBVIEW2_USER_DATA_DIR
- Nothing written to AppData, registry, or any system location
- Update README with portable usage info

src-tauri/tauri.conf.json

@@ -30,7 +30,7 @@
   },
   "bundle": {
     "active": true,
-    "targets": "all",
+    "targets": [],
     "icon": [
       "icons/32x32.png",
       "icons/128x128.png",