refactor: architecture cleanup — OIDC dedup, PKCE, K8s hardening

- Extract OidcProviderHelper for shared discovery + JWK source construction
- Add SystemRole.normalizeScope() to centralize role normalization
- Merge duplicate claim extraction in OidcTokenExchanger
- Add PKCE (S256) to OIDC authorization flow (frontend + backend)
- Add SecurityContext (runAsNonRoot) to all K8s deployments
- Fix postgres probe to use $POSTGRES_USER instead of hardcoded username
- Remove default credentials from Dockerfile
- Extract sanitize_branch() to shared .gitea/sanitize-branch.sh
- Fix sidebar to use /exchanges/ paths directly, remove legacy redirects
- Centralize basePath computation in router.tsx via config module

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

ui/src/auth/LoginPage.tsx

@@ -11,6 +11,22 @@ interface OidcInfo {
   authorizationEndpoint: string;
 }
 
+/** Generate a random code_verifier for PKCE (RFC 7636). */
+function generateCodeVerifier(): string {
+  const array = new Uint8Array(32);
+  crypto.getRandomValues(array);
+  return btoa(String.fromCharCode(...array))
+    .replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
+}
+
+/** Derive the S256 code_challenge from a code_verifier. */
+async function deriveCodeChallenge(verifier: string): Promise<string> {
+  const data = new TextEncoder().encode(verifier);
+  const digest = await crypto.subtle.digest('SHA-256', data);
+  return btoa(String.fromCharCode(...new Uint8Array(digest)))
+    .replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
+}
+
 const SUBTITLES = [
   "Prove you're not a mirage",
   "Only authorized cameleers beyond this dune",
@@ -68,14 +84,20 @@ export function LoginPage() {
     if (oidc && !forceLocal && !autoRedirected.current) {
       autoRedirected.current = true;
       const redirectUri = `${window.location.origin}${config.basePath}oidc/callback`;
-      const params = new URLSearchParams({
-        response_type: 'code',
-        client_id: oidc.clientId,
-        redirect_uri: redirectUri,
-        scope: 'openid email profile',
-        prompt: 'none',
+      const verifier = generateCodeVerifier();
+      sessionStorage.setItem('oidc-code-verifier', verifier);
+      deriveCodeChallenge(verifier).then((challenge) => {
+        const params = new URLSearchParams({
+          response_type: 'code',
+          client_id: oidc.clientId,
+          redirect_uri: redirectUri,
+          scope: 'openid email profile',
+          prompt: 'none',
+          code_challenge: challenge,
+          code_challenge_method: 'S256',
+        });
+        window.location.href = `${oidc.authorizationEndpoint}?${params}`;
       });
-      window.location.href = `${oidc.authorizationEndpoint}?${params}`;
     }
   }, [oidc, forceLocal]);
 
@@ -86,15 +108,20 @@ export function LoginPage() {
     login(username, password);
   };
 
-  const handleOidcLogin = () => {
+  const handleOidcLogin = async () => {
     if (!oidc) return;
     setOidcLoading(true);
     const redirectUri = `${window.location.origin}${config.basePath}oidc/callback`;
+    const verifier = generateCodeVerifier();
+    sessionStorage.setItem('oidc-code-verifier', verifier);
+    const challenge = await deriveCodeChallenge(verifier);
     const params = new URLSearchParams({
       response_type: 'code',
       client_id: oidc.clientId,
       redirect_uri: redirectUri,
       scope: 'openid email profile',
+      code_challenge: challenge,
+      code_challenge_method: 'S256',
     });
     window.location.href = `${oidc.authorizationEndpoint}?${params}`;
   };