fix(linux): fcitx5 hotkey intercept + remove backdrop-filter + clipboard cleanup
- fcitx5 plugin: fix Alt+a hotkey leaking 'a' character by normalizing
case comparison (Alt produces uppercase sym, Key::check expects lowercase)
- fcitx5 plugin: use PreInputMethod + filterAndAccept to block engine
before commit happens (same strategy as clipboard addon)
- Linux: remove backdrop-filter (unreliable on WebKitGTK, #553),
replace with opaque background
- Linux: skip clipboard writes after streaming insert (fcitx5 commits
directly, clipboard fallback corrupts user data)
- SettingsModal: render via React Portal to document.body (#580, fixes
scrollbars overlapping modal)
- Remove is_no_compositing_mode command and CSS fallback (no longer needed)
- Remove StartupShell loading screen (app renders FloatingShell immediately)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
