Draft: Explore a modernized desktop client UI

[SebastianBoehler]

Summary

This draft PR is a discussion/testing branch for #814. It explores a more modern desktop-client look by applying a shallow styling and layout refresh rather than a full UI/UX redesign.

Changes included here:

• split the theme QSS into src/cfclient/utils/themes.py so the Default, Navy, and Hacker themes share the same modern base styling
• modernized common controls, tabs, menus, inputs, combo boxes, and spin boxes across themes
• aligned the top connection row controls to the same height
• adjusted the Flight Control tab input widths, label spacing, and command-pad area
• added a padded macOS app icon variant to avoid the menu-bar icon appearing oversized
• added a Default-theme screenshot under docs/images/ for visual context

This is intentionally not a complete redesign. It mostly overwrites styling at the top level and makes a few targeted layout adjustments so there is something concrete to test and discuss with the interface maintainers.

Scope notes

• No scanner/connect behavior changes are included.
• The branch should be treated as inspiration and a review surface for the design direction, not as a claim that the full client UX has been reworked.
• The screenshot in docs/images/cfclient-default-theme-refresh.png shows the current sample implementation.

Checks

• python -m compileall -q src/cfclient/utils/themes.py src/cfclient/utils/ui.py src/cfclient/gui.py
• Qt offscreen uic.loadUiType smoke checks for src/cfclient/ui/main.ui and src/cfclient/ui/tabs/flightTab.ui
• theme smoke test for Default, Navy, Hacker, and unused theme entries
• UV_PROJECT_ENVIRONMENT=/tmp/cfclient-codex-venv uv run --frozen cfclient --check-imports
• git diff --check