fix(auth): restore /echo prefix after proxy strips it from next param

The reverse proxy strips /echo/ before Python, so next=/workspace.html.
Both the JS redirect and the server-side already-logged-in path now
prepend /echo to produce a valid public URL.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dashboard/api.py

@@ -10,7 +10,7 @@ import os
 import sys
 from http.server import SimpleHTTPRequestHandler, ThreadingHTTPServer
 from pathlib import Path
-from urllib.parse import quote as _urlquote
+from urllib.parse import quote as _urlquote, parse_qs, urlparse
 
 # Make dashboard/ importable for the handler submodules (constants,
 # habits_helpers, handlers.*). Tests rely on this as well.
@@ -243,11 +243,19 @@ class TaskBoardHandler(
             return
         elif self.path in ('/echo/login', '/login') or \
                 self.path.startswith(('/echo/login?', '/login?')):
-            # If already logged in, redirect to workspace; otherwise serve
+            # If already logged in, redirect to next (or workspace); otherwise serve login.html.
-            # login.html (created in Lane B2).
             if self._check_dashboard_cookie():
+                qs = parse_qs(urlparse(self.path).query)
+                next_vals = qs.get('next', [])
+                nxt = next_vals[0] if next_vals else ''
+                # Proxy strips /echo/ before Python, so nxt is e.g. /workspace.html.
+                # Re-add the prefix so the browser lands on the right public URL.
+                if nxt and nxt.startswith('/') and '://' not in nxt:
+                    dest = '/echo' + nxt
+                else:
+                    dest = '/echo/workspace.html'
                 self.send_response(302)
-                self.send_header('Location', '/echo/workspace.html')
+                self.send_header('Location', dest)
                 self.send_header('Content-Length', '0')
                 self.end_headers()
                 return

dashboard/login.html

@@ -257,8 +257,11 @@
                         // relative /echo/ path to prevent open-redirect attacks.
                         var params = new URLSearchParams(window.location.search);
                         var next = params.get('next') || '';
-                        var dest = (next && /^\/echo\/[^/]/.test(next) && next.indexOf('/echo/login') !== 0)
+                        // The proxy strips /echo/ before Python, so `next` is
-                            ? next
+                        // e.g. "/workspace.html". Re-add the /echo prefix for
+                        // the browser. Guard against open-redirect (no ://).
+                        var dest = (next && /^\/[^/]/.test(next) && next.indexOf('://') === -1)
+                            ? '/echo' + next
                             : '/echo/workspace.html';
                         window.location.assign(dest);
                         return;