Add unsupported notification for YouTube / Google Drive Video URls

## Problem

Unity client allows scenes to use YouTube and Google Drive video URLs via a method that violates YouTube's Terms of Service. This cannot be maintained in the mobile client.

**Current behavior on mobile:**
- YouTube/Google Drive video URLs show as black/nothing
- Works on Unity desktop
- Users will expect feature parity and complain ("NO FUNCA EL VIDEO Y EN DESKTOP SI")

## Technical Context

- Videos are rendered as textures sent to the GPU
- Creators have full control over aspect ratios and placement (could be 16:9, 9:16, 45:1, spheres, etc.)
- Textures can be applied to any shape, split across multiple screens, or cropped
- Cannot prevent deformation without changing the approach entirely

## Proposed Solutions

### Option 1: Placeholder Texture
Display a specific texture/image when YouTube/Google Drive URLs are detected (reference image was shared in Slack).

**Challenges:**
- May appear deformed depending on creator's aspect ratio choices
- Could implement on-the-fly image generation but costs performance
- Loses control once texture is sent to GPU

### Option 2: In-World Notification
Show a notification when user is near an unsupported video source.

### Option 3: Creator-Side Warnings
- Creator Hub warnings when unsupported URLs are used
- Documentation updates
- Foundation discussion needed

## Acceptance Criteria

- [ ] Determine elegant user feedback approach (placeholder, notification, or other)
- [ ] Design solution that handles arbitrary aspect ratios gracefully
- [ ] Implement chosen solution for production (not just preview)
- [ ] Consider Creator Hub warnings to prevent usage
- [ ] Coordinate with Foundation on messaging/policy

## Notes

- This affects **production**, not just preview environment
- Need to avoid user complaints about desktop vs mobile parity
- Elizabeth to design if an elegant solution can be found

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Requested by Mateo via Slack