Fix desktop viewport resizing and define minimum window size

[Cheggin]

Problem

Resizing the desktop app window is currently awkward because the embedded browser/viewport appears to keep its old size instead of adapting to the new app bounds. This makes the app feel broken at smaller or resized dimensions.

We also need a sensible minimum window size so the UI does not collapse into unusable layouts.

Scope

• Investigate why the browser viewport does not resize with the desktop app window.
• Ensure browser content, chrome, side panels, and agent UI respond correctly when the window is resized.
• Define and enforce minimum desktop window dimensions for each supported platform.
• Clamp or adapt layouts below the minimum so the app does not look broken.
• Verify behavior when entering/exiting fullscreen and when moving between displays with different scale factors.

Acceptance criteria

• Resizing the desktop app resizes the browser viewport and related UI surfaces in sync.
• The app enforces a minimum window size that keeps primary workflows usable.
• No major UI elements overlap or disappear at the minimum size.
• Browser content remains visible and interactive after repeated resize events.
• Regression coverage exists for resize handling, and manual QA notes cover at least macOS plus one non-macOS target if available.