Fix auth dialog URL construction for subpath hosting

window.location.origin alone drops the /usable-excel/ subpath on GitHub
Pages, causing the dialog to open at a 404 URL and the Keycloak redirect
to point to the wrong origin.

- use-auth.ts: derive basePath from window.location.pathname so the
  dialog URL includes the correct subpath in any hosting environment
- auth-dialog.html: use window.location.pathname for REDIRECT_URI instead
  of hardcoded /auth-dialog.html, same reason

Both fixes are dev/prod agnostic — work on localhost:3333 and GitHub Pages.

Co-Authored-By: Claude (claude-sonnet-4-6) <noreply@anthropic.com>

Author: flowcore-platform

src/taskpane/auth-dialog.html

@@ -56,7 +56,7 @@ <h2 id="title">Signing in…</h2>
       var CLIENT_ID = 'mcp_oauth_client';
       var SCOPES    = 'openid profile email offline_access';
       // Redirect back to this same page (handles both start and callback)
-      var REDIRECT_URI = window.location.origin + '/auth-dialog.html';
+      var REDIRECT_URI = window.location.origin + window.location.pathname;
 
       /* ------------------------------------------------------------------ */
       /* Helpers                                                              */

src/taskpane/hooks/use-auth.ts

@@ -161,7 +161,8 @@ export function useAuth(): AuthResult {
   // -------------------------------------------------------------------------
 
   const login = useCallback(() => {
-    const dialogUrl = `${window.location.origin}/auth-dialog.html`;
+    const basePath = window.location.pathname.substring(0, window.location.pathname.lastIndexOf('/') + 1);
+    const dialogUrl = `${window.location.origin}${basePath}auth-dialog.html`;
 
     Office.context.ui.displayDialogAsync(
       dialogUrl,