webui: support video files as input

[foldl]

Overview

Support adding video files as input. This can fix #20741.

Everything is done almost the same as audio files.

Detailed Modifications

1. Add a menu item for uploading video files;
2. Show an icon in the chat input box (ChatAttachmentsListItemThumbnailFile) like ChatAttachmentsPreviewThumbnailStrip;
3. A new preview window for video files;
4. Video files are sent to the server through input_video (just like input_audio for audio files);
5. Two types of video files are defined (mp4 and ogg);
6. On Model Information window, video modality is shown as "Vision (Video)", and the vision modality is shown as "Vision (Image)";
7. Add a new bool field video to Modalities.

Test & Sceenshots

I have tested this with chatllm.cpp.

Additional information

Some findings or thoughts that are out of the scope of this PR.

• How to properly show the modalities of image-only, and image-video?
• Video files often contain audio. At present, when sending to servers, media types are inferred from file extension but not the menu item which is clicked by users.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: NO. I coded this all by myself (copied and modified some existing codes).

[ngxson]

IMO this can be an acceptable stop-gap solution. But just one concern is that we will eventually have native video support in mtmd, so we should make sure changes from this PR can be easily reverted when it happens.

[foldl]

This is for mtmd (see the issue #20741). Why this needs to be reverted when video support in mtmd is ready?

[allozaur]

Please rebase this on latest commit on master and solve conflicts.

[ServeurpersoCom]

LGTM, video has been added symmetrically to image and audio.
If refactor is required, all three can be maintained in the same way.