[ATLAS-312] feat: add Gofile host with self-healing salt rotation

[codeon89]

What this changes

• Adds Gofile as a download host. One-file folders queue straight from the update modal; multi-file folders open a picker; Settings shows free usage (1 TB / 30 days) with no login.

Why

#312

• Gofile (non-premium) login support

How it was tested

• Added tests that cover this change
• For a fix: the regression test fails against the unfixed code
• npm run check passes locally
• New functions and IPC handlers have comments explaining why
• CHANGELOG.md updated
• This PR targets nightly, not main

AI assistance

• No AI was used on this change
• AI was used on this change

Tool and model: VSCode auto-free

What it wrote:

• initial draft for gofile.js (heavily modified to fix issue while testing)
• scripts/check-host-plugins.js
• All tests
• Comments

What you verified yourself:

• Verify download with Gofile.
• Manual testing with changes to fix API calls to prevent server block from Gofile.

IPC changes

• downloads-list-folder. Handler in electron/ipc/downloads.js, exposed in preload.js, called in UpdateModal.jsx

Anything the reviewer should know

• Purposely ignore "password-protected" use case and "premium use case" - one is unlikely for site download so not needed for now, and premium API key will be another time since the approach will be slightly different.