Appeal page shows a blank page or no message when login or the ban check fails #294

Files: src/pages/appeal.tsx (exchangeCodeForToken at line 124, checkBan, and the render branches from line 554)

Problem. Two failure paths leave the user with nothing to go on:

• Failed login. If /api/v2/keycloak/token fails or returns no access_token, the catch at line 152 ignores it. The page falls back to the "log in" screen with no message, and ?code=… stays in the URL. Before Pass the Keycloak client ID to next build (WEB-031) #266, every ban-appeal login failed exactly like this, and users had no way to tell.
• Failed ban check. If any call in checkBan() fails (/users/banned, or getting a token), banStatus becomes "unknown". The page only renders branches for can_appeal, can_reappeal, has_appeal and not_banned, so a logged-in user sees the header and footer with an empty page in between.

Separately, the alert colour is chosen by message.includes("submitted") || message.includes("success"), so a server message worded differently shows as an error.

Fix.

• Show an error with a retry or "log in again" button for both paths, and remove the spent code from the URL.
• Add a branch for token && banStatus === "unknown".
• Track success and error explicitly instead of matching on message text.

Done when. With the token or ban endpoint returning 500, the page explains what went wrong and offers a next step.

Related: #208 (ban appeal fixes).