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/capabilities/default.json

@@ -13,8 +13,6 @@
     "core:window:allow-start-dragging",
     "fs:default",
     "fs:allow-read-text-file",
-    "fs:read-all",
-    "window-state:allow-restore-state",
-    "window-state:allow-save-window-state"
+    "fs:read-all"
   ]
 }