Fix gallery play page layout overflow and show action buttons for guests

[SSoggyTacoMan]

Supersedes #4076. Fixes #3467.

Summary of changes

1. Fix player layout overflow on smaller displays (e.g. 1366x768):

   • In desktop-player.module.css, replaced position: fixed with responsive flex container sizing.
   • Constrained canvas scaling with max-height: min(800px, calc(100vh - 250px)) in embedded mode and width: auto, maintaining aspect ratio while ensuring the canvas and controls fit comfortably within the viewport.
   • Removed position: absolute on .screenControls so controls naturally position directly below the canvas and match its width.
   • Added native :fullscreen styles so fullscreen scaling works reliably when requested.
   • Added an embedded prop to DesktopPlayer to cleanly skip rendering the unused metadata sidebar (#desktop-player-meta).

2. Show action buttons for logged-out visitors:

   • In MainNavbar, rendered props.children in the logged-out branch instead of dropping custom actions, allowing guests to use "View Code" and "Remix Game".

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