Light mode titlebar and toolbar to follow the theme selected

### The feature request

Currently, GitHub Desktop's title bar (window chrome) and toolbar (the row containing the Current Repository, Current Branch, and
Push/Pull buttons) use hardcoded dark colors regardless of the user's selected theme. Switching to the light theme changes the rest
of the UI but leaves these two surfaces dark with white text, which is visually inconsistent and feels unfinished.

Expected behavior:
- In light theme, the title bar and toolbar should use a light background with dark text.
- In dark theme, they should remain dark with light text (current behavior).
- Window control buttons (minimize, maximize, close) and hover/active states should adapt accordingly.

Why it matters:
Users who deliberately choose the light theme expect the entire application to follow it. The current behavior makes the top of the
window look like it belongs to a different app, and reduces the perceived polish of an otherwise carefully themed UI.

Platform: Windows (macOS uses a hardcoded gradient that this request does not address; that could be a follow-up).

### Proposed solution

### Additional context

_No response_

Start by tracing the Windows title bar and toolbar rendering from the existing theme-selection behavior, then identify where their hardcoded dark colors and window-control states are defined. Done means light theme uses light surfaces and dark text, while dark theme preserves the current appearance and hover/active states and window controls adapt accordingly.

Written by the indexing model from the issue text.