feat(preview): add support for mkv video files - #3564

Description

Registers video/x-matroska and video/matroska in the Preview app so .mkv files can be opened in it. Same approach as #1705 (MOV). Playback relies on native browser support.

Related Issue

• Fixes Add support for .mkv (Matroska) files in the preview #3563

How Has This Been Tested?

• test environment: local OpenCloud (docker compose) with a local build, Chromium, Firefox and GNOME Web (as a stand-in for Safari, WebKit based)
• test case 1: "Open with > Preview" is offered for .mkv and a click on the file opens the viewer
• test case 2: H.264+AAC and VP9+Opus files play in all three browsers
• test case 3: public link to an .mkv opens and plays (Firefox)
• not tested: real Safari

Types of changes

• Bugfix
• Enhancement (a change that doesn't break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)