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.lock

@@ -3703,21 +3703,6 @@ dependencies = [
  "zbus",
 ]
 
-[[package]]
-name = "tauri-plugin-window-state"
-version = "2.4.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "73736611e14142408d15353e21e3cca2f12a3cfb523ad0ce85999b6d2ef1a704"
-dependencies = [
- "bitflags 2.10.0",
- "log",
- "serde",
- "serde_json",
- "tauri",
- "tauri-plugin",
- "thiserror 2.0.18",
-]
-
 [[package]]
 name = "tauri-runtime"
 version = "2.10.0"
@@ -4305,7 +4290,6 @@ dependencies = [
  "tauri-plugin-dialog",
  "tauri-plugin-fs",
  "tauri-plugin-opener",
- "tauri-plugin-window-state",
 ]
 
 [[package]]