Update TASKS.md and add TODO comments for unfinished features

[google-labs-jules]

Scanned the codebase for indications of unfinished work.

• Updated TASKS.md with tasks for Subscription endpoint, Gallery features, Import tabs, Resumable downloads, and DownloaderFactory tests.
• Added explicit TODO comments in Octans.Client/Endpoints.cs, Octans.Client/Components/Imports/Imports.razor, Octans.Client/Components/Gallery/Gallery.razor, and Octans.Core/Downloads/DownloadService.cs to track these tasks in the code.

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PR created automatically by Jules for task 965444445458914422 started by @jkendall327

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