feat: redirect to /settings with error banner when Gitea API token is expired

Add isTokenError() helper that detects HTTP 401/403 responses from the
Gitea API, and redirectOnTokenError() that redirects to /settings with
an error=token_expired query parameter. Update Dashboard, ListIssues,
and ListPulls handlers to check for token errors. The settings page now
displays an error banner explaining the token needs to be refreshed.

Closes leeworks-agents/gitea-mobile#192

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/handlers/settings.go

@@ -45,6 +45,13 @@ func (h *SettingsHandler) handleGet(w http.ResponseWriter, r *http.Request) {
 	}
 
 	data := settingsData{HasToken: hasToken}
+
+	// Show error banner when redirected due to expired/revoked token.
+	if r.URL.Query().Get("error") == "token_expired" {
+		data.Message = "Your Gitea API token is expired or has been revoked. Please enter a new token."
+		data.MessageType = "error"
+	}
+
 	h.renderSettings(w, data)
 }