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

.gitignore

@@ -19,6 +19,9 @@ src-tauri/target/
 icon-*.html
 nul
 
+# Portable data directory
+data/
+
 # Editor directories and files
 .vscode/*
 !.vscode/extensions.json