Retry a failed image fetch instead of giving up for the session

[ShaalaXIV]

A portrait that would not stay put between game restarts was never a saving problem. The picture was on the server the whole time.

ImageCache set Failed on the first unsuccessful fetch and never cleared it:

if (entries.TryGetValue(key, out var existing))
{
    existing.LastTouched = DateTime.UtcNow;
    return existing.Texture;   // null for ever, once failed
}

So an image that failed once was dead until the plugin reloaded. The commonest cause is the card being drawn before the plugin has finished connecting, or a blink of network at login — half a second of bad luck turning into a picture that never appears again that session. Restarting the game built a fresh cache, which is exactly why it looked like the photo failed to persist and then came back.

Changes

• Failed fetches retry with backoff: 2, 4, 8, 16, 32, then 60 seconds. A success resets the count.
• OperationCanceledException during unload is no longer recorded as a failure — it poisoned an entry for a cache about to be discarded.
• A Loading flag prevents a retry starting a second concurrent fetch.
• The frame stops lying: it said "Loading..." whether a fetch was in flight or had given up. After three failures it now says "Picture unavailable". Both new labels are covered by the contrast audit.

🤖 Generated with Claude Code