fix(preview): let arrow keys seek video instead of switching files

[Brumaire]

Description

When previewing a video in a folder containing more than one file, pressing the left/right arrow keys switches to the previous/next file instead of seeking the video. This makes it impossible to use the keyboard to seek videos in any folder with multiple items — single-video folders work because there is nothing to switch to, so the event falls through to video.js.

The root cause is in frontend/src/views/files/Preview.vue: the key handler unconditionally maps ArrowLeft / ArrowRight to prev() / next() without checking the current file type, so video.js never gets a chance to handle them.

Additional Information

This PR adds a small guard: when the current preview is a video (fileStore.req?.type === "video"), ArrowLeft and ArrowRight return early and the event falls through to video.js for seeking. Enter is preserved as an "advance to next file" shortcut so multi-video folders still have a keyboard way to switch, and Esc is unchanged.

Behavior summary:

Key	Non-video preview	Video preview (after this PR)
ArrowLeft	prev file	video.js seek backward
ArrowRight	next file	video.js seek forward
Enter	next file	next file (unchanged)
Esc	close	close (unchanged)

Related discussion: #4908 (video player improvements).

Checklist

• I am aware the project is currently in maintenance-only mode.
• I am aware that translations MUST be made through Transifex and that this PR is NOT a translation update.
• I am making a PR against the master branch.
• I am sure File Browser can be successfully built. (Built and verified locally; the patched image has been running healthy on my own deployment.)

[hacdias]

Small and nice fix!