feat(windows): persist size + position across restarts via tauri-plugin-window-state

Without this, every secondary window (preview overlay, task float,
transcript viewer) opened at whatever spot Tauri / the compositor picked,
which was especially noticeable on Wayland where placement hints are
advisory. Main window's position was also lost on restart.

Registering tauri_plugin_window_state in the builder gives automatic
per-window-label save + restore. State lives in app-data/window-state.json;
fresh installs still fall back to the builder defaults (no changes to
inner_size on any of the four windows). Covers OpenWhispr issue #605 and
the broader UX pain.

No frontend changes — the plugin is purely backend. Regenerated ACL
manifests / desktop + linux schemas pick up the plugin registration.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -29,6 +29,7 @@ tauri-plugin-opener = "2"
 tauri-plugin-dialog = "2"
 tauri-plugin-global-shortcut = "2"
 tauri-plugin-updater = "2"
+tauri-plugin-window-state = "2"
 
 # Serialisation
 serde = { version = "1", features = ["derive"] }