Add blitz feature for Dioxus Native rendering support

Add an orthogonal `blitz` feature flag that switches the rendering
framework from dioxus webview to dioxus-native (Blitz). Can be combined
with `desktop` or `mobile` features. Also bumps rfd to ^0.17.0 (upstream
git moved forward), removes stale gui-level [patch.crates-io] section,
and adds xdotool to the Dockerfile for libxdo (blitz-shell dependency).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gui/assets/icons/error.svg

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