feat: 404 pages → redirect to /workspaces

- main.py: @app.exception_handler(404) → RedirectResponse(/workspaces)
- dashboard.py: page_detail 404 → RedirectResponse(/workspaces)
- tests: test_styled_404_page → 302 redirect
- API paths (containing /api) still get JSON 404, not redirect
- 143 tests passent

tests/test_app.py

@@ -1950,11 +1950,10 @@ def test_create_page_defaults_to_untitled(client):
 
 
 def test_styled_404_page(client):
-    """Unknown routes return a styled 404 HTML page."""
+    """Unknown routes redirect to /workspaces."""
     resp = client.get("/this-does-not-exist-xyz", follow_redirects=False)
-    assert resp.status_code == 404
-    assert "404" in resp.text
-    assert "FlowDeck" in resp.text
+    assert resp.status_code == 302
+    assert resp.headers.get("location") == "/workspaces"
 
 
 def test_api_404_returns_json(client):