feat: add popup notification system for texture loading errors

[coderabbitai]

Summary

When the normal map texture (/normal-map.png) fails to load in the globe component, the globe currently hangs in the wireframe/loading state with no user feedback.

A popup notification system should be implemented to inform the user when this error occurs and prompt them to reload the page.

Context

• Raised during review of PR feature: add 3d rotatable globe #18 (comment: feature: add 3d rotatable globe #18 (comment))
• Requested by @dastarruer

Acceptance Criteria

• Implement a reusable popup/toast notification component
• Show a popup notification when useTexture('/normal-map.png') fails to load (i.e., in the .catch handler of the promise in app/src/lib/components/globe/Globe.svelte)
• The notification should inform the user of the error and prompt them to reload the page
• The popup system should be general enough to be reused for other error scenarios in the future