re-implemented login via OIDC

frontend/src/routes/user/OidcCallback.svelte

@@ -0,0 +1,33 @@
+<script>
+    import { onMount } from 'svelte';
+    import { t } from '../../translations.svelte.js';
+    import { useTinyRouter } from 'svelte-tiny-router';
+    import { checkUser } from '../../user.svelte.js';
+
+    const router = useTinyRouter();
+
+    let message = $state(t('user.processing_code'));
+    onMount(async () => {
+        let params = new URLSearchParams(location.search);
+
+        if (params.get('code')){
+            const url = `${location.protocol}//${location.host.replace('5173','8080')}/api/user/oidc/token`;
+            const resp = await fetch(url,{
+                method : 'POST',
+                body: JSON.stringify(Object.fromEntries(params)),
+                credentials: 'include'
+            });
+            if (resp.ok){
+                let json = await resp.json();
+                const redirect = json.redirect ? json.redirect : '/user';
+                checkUser();
+                router.navigate(redirect);
+            } else {
+                message = await resp.text();
+                if (!message) message = t(resp);
+            }
+        }
+    });
+</script>
+
+{message}