feat: Avoiding duplicates when importing same theme URL again

[Arcitec]

Feature description

This is related to #771.

The idea is as follows:

• Remember the upstream URL whenever a theme is imported via URL.
• If that URL is imported again, tell the user something like "Theme XYZ from (url) was already imported. Replace the current local copy?" or somethinbg. To avoid duplicates when importing same theme again.
• Similar code would also be involved in a potential, future "Check original URL for updates, then auto-download and replace local theme if an update exists" feature. So having the core "detect identical URL and replace local copy" function as a modular piece would help make that a reality someday too.

Additional Context

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