feat(forum): add spoiler tags and YouTube/SoundCloud embeds

[quantbitrealmSimon]

Description

This PR implements enhanced forum content features as requested in #46.

Features Added

1. Spoiler Tags - Users can now use ||spoiler text|| syntax to hide content behind clickable spoilers

   • Click to reveal/hide spoiler content
   • Smooth transition animation
   • Clear UI indication

2. YouTube Video Embeds - Auto-embed YouTube videos from:

   • Direct syntax: youtube:VIDEO_ID
   • URLs: https://youtube.com/watch?v=VIDEO_ID or https://youtu.be/VIDEO_ID
   • Responsive iframe with 560x315 dimensions

3. SoundCloud Embeds - Embed SoundCloud tracks using:

   • Syntax: soundcloud:TRACK_URL
   • Full player with artwork, comments, and controls

Implementation

• Created EnhancedMarkdown.jsx component extending ReactMarkdown
• Custom text renderer processes placeholders for spoilers and embeds
• Added comprehensive CSS styling in markdown.css
• Updated Thread.jsx to use the new component

Usage Examples

This is a ||spoiler|| that you can click to reveal!

Check out this video: youtube:dQw4w9WgXcQ

Listen to this: soundcloud:https://soundcloud.com/artist/track

Testing

• Spoiler tags render correctly and toggle on click
• YouTube videos embed with correct dimensions
• SoundCloud tracks embed with full player
• Multiple embeds/spoilers in same post work correctly
• Existing markdown content continues to work

Screenshots

(Screenshots would be attached here showing the features in action)

Fixes #46

/claim 46

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/cc @pikulet