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/Cargo.toml

@@ -19,7 +19,6 @@ tauri = { version = "2", features = [] }
 tauri-plugin-opener = "2"
 tauri-plugin-dialog = "2"
 tauri-plugin-fs = "2"
-tauri-plugin-window-state = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"