Fix: picking Sequoia Trail or Pismo Beach still showed Golden Gate

[DevMando]

Summary

In Settings → Appearance, choosing the Sequoia Trail or Pismo Beach background tile appeared to do nothing — the chat kept showing Golden Gate no matter which tile was selected. This PR fixes that with a one-line change.

What was wrong

When a background is picked, the app copies the image into the user's data folder and tells the chat view to load it with a small version stamp on the URL, so the view knows to fetch the new picture instead of reusing an old one. That version stamp comes from the image file's timestamp — but the copy step preserves the original file's timestamp, and all three bundled backgrounds happen to share the exact same one (they were written to disk in a single download). As a result, every gallery pick produced an identical URL, and the chat view kept serving the first image it had cached: Golden Gate. The selection itself was being saved correctly — only the on-screen picture was stale.

What changed

After copying the chosen image, the app now stamps the copy with the current time. Every pick therefore gets a fresh URL and the chat view fetches the right picture immediately. This also protects user-supplied images that could hit the same coincidence.

Scope and risk

• One file changed (ThemeManager.cs), five lines added, no behavior changes beyond the fix.
• No settings format changes; existing saved backgrounds keep working.
• Builds clean with 0 warnings / 0 errors.

How to verify

1. Open Settings → Appearance.
2. Click Sequoia Trail — the preview and open chats should switch immediately.
3. Click Pismo Beach, then back to Golden Gate — each should apply on the first click.
4. Restart the app — the last pick should still be shown and its tile marked selected.