fix(templates): persist workspace context so template pages actually save

Pages created via the page-template picker (weekly report, project doc,
meeting notes, ...) previously got workspace_id=NULL and workspace=login,
so they never appeared in the active local/Gitea workspace tree — they
looked like they 'didn't save' even though the row existed.

Frontend: _useTemplate now sends the active workspace context (workspace_id
or workspace key), mirroring _createPlainPage.
Backend: use_page_template resolves/validates the workspace and persists
both workspace and workspace_id on the new page.

tests/test_v511_v512_wiki_templates.py

@@ -283,6 +283,39 @@ def test_use_builtin_template_creates_page(client):
                        cookies={"flowdeck_session": s}, headers={"X-CSRF-Token": c}).status_code == 404
 
 
+def test_use_template_persists_workspace_id(client):
+    s, c, uid = _login(client)
+    # Create a local workspace owned by this user
+    r = client.post("/api/workspaces", json={"name": "Tpl WS"},
+                    cookies={"flowdeck_session": s}, headers={"X-CSRF-Token": c})
+    assert r.status_code == 200
+    ws_id = r.json()["id"]
+    client.post(f"/api/workspaces/{ws_id}/select", cookies={"flowdeck_session": s})
+    # built-in template use with workspace context
+    r2 = client.post("/board/api/page-templates/0/use", json={"key": "meeting_notes", "workspace_id": ws_id},
+                     cookies={"flowdeck_session": s}, headers={"X-CSRF-Token": c})
+    assert r2.status_code == 200
+    pid = r2.json()["id"]
+    page = client.get(f"/board/api/pages/{pid}").json()
+    assert page["workspace_id"] == ws_id
+    assert page["workspace"] == "Tpl WS"
+    # user template use with workspace context
+    r3 = client.post("/board/api/page-templates",
+                     json={"name": "Tpl WS template", "blocks": _blocks_b("ws body")},
+                     cookies={"flowdeck_session": s}, headers={"X-CSRF-Token": c})
+    tid = r3.json()["id"]
+    r4 = client.post(f"/board/api/page-templates/{tid}/use", json={"workspace_id": ws_id},
+                     cookies={"flowdeck_session": s}, headers={"X-CSRF-Token": c})
+    assert r4.status_code == 200
+    page2 = client.get(f"/board/api/pages/{r4.json()['id']}").json()
+    assert page2["workspace_id"] == ws_id
+    assert page2["workspace"] == "Tpl WS"
+    # routing: template-created page must appear in the workspace tree
+    tree = client.get("/api/local-workspace/tree", cookies={"flowdeck_session": s}).json()
+    ids = [n["id"] for n in tree["tree"]]
+    assert pid in ids
+
+
 def test_user_template_roundtrip(client):
     s, c, uid = _login(client)
     pid = _mk_page(client, s, "My Weekly", _blocks_b("body text"))