[Desktop] "Check for updates" spinner does not resolve Linux

**Description**

On Linux desktop installs (deb/rpm/flatpak), clicking Check for Updates in Settings → App leaves the spinner running infinitely ("Checking for updates..."). The check itself completes instantly on the main process with a null result and no error, but the renderer never transitions out of the checking state.

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**Steps To Reproduce**
1. Install the official desktop package on Linux (Ubuntu 24.04 aarch64, goose_1.48.0_arm64.deb)
2. Open Settings → App
3. Click Check for Updates

**Observed Behavior**
* Spinner keeps going indefinitely
Log signature (from ~/.config/Goose/logs/main.log):

```[2026-08-29 01:46:19.193] [info] === MANUAL UPDATE CHECK INITIATED ===
[2026-08-29 01:46:19.193] [info] Current version: 1.48.0
[2026-08-29 01:46:19.194] [warn] APPIMAGE env is not defined, current application is not an AppImage
[2026-08-29 01:46:19.194] [info] === MANUAL UPDATE CHECK COMPLETED in 1ms ===
[2026-08-29 01:46:19.194] [info] Auto-updater checkForUpdates result: null
```
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**Expected behavior**
Check either reports "up to date" (when on latest), or detects an available update (via the GitHub fallback) and offers to download/install. Either way the user should get some sort of feedback in < 10s. Checking for updates should be fast on anything but the slowest connection.

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**Screenshots**
If applicable, add screenshots to help explain your problem.

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**Please provide the following information**
- OS & Arch: Ubuntu 24.04.4 LTS, arm64 (aarch64) & Pop!_OS 24.04 LTS
- Interface: UI (Electron desktop)
- Version: v1.48.0 (also observed on v1.46.0 and v1.45.0)
- Extensions enabled: n/a
- Provider & Model: n/a
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**Additional context**
Add any other context about the problem here.

Do not begin implementation until the issue reaches **Ready** on the [Goose Issues board](https://github.com/orgs/aaif-goose/projects/1).