feat(media): add fullscreen keyboard shortcut to React controls

[ittaishay]

Summary

Adds the 'f' keyboard shortcut to toggle fullscreen mode in the React video controls (handleKeydownReact), matching the existing behavior in the legacy controls (handleKeydown).

Changes

• Added f and shift+f key handlers to handleKeydownReact in MediaBaseViewer.js
• Added corresponding unit tests in MediaBaseViewer-test.js

Background

The video player has two keyboard handling paths:

• Legacy controls (handleKeydown) - already supported 'f' for fullscreen
• React controls (handleKeydownReact) - was missing this shortcut

This PR brings parity between the two by adding the fullscreen shortcut to the React controls path.

Test Plan

• Press 'f' while viewing a video with React controls enabled - should toggle fullscreen
• Press 'Shift+F' while viewing a video - should also toggle fullscreen
• Verify existing keyboard shortcuts still work (space/k for play/pause, arrow keys for seek, m for mute, etc.)

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