Fix: "Clear my history" now clears account history too

clearSession only reset the device-local history; for a signed-in user the panel
shows the account history (serverHistory), which was never cleared and never sent
to the server — so it looked like nothing happened. Add DELETE /api/history
(clear all) and have clearSession reset serverHistory + call it when signed in.

tests/test_saved_api.py

@@ -70,3 +70,6 @@ def test_history_and_import(client):
     # granular removal from history
     tc.delete("/api/history/2")
     assert {a["id"] for a in tc.get("/api/history").json()["items"]} == {1, 3}
+    # clear ALL history
+    assert tc.delete("/api/history").json() == {"ok": True}
+    assert tc.get("/api/history").json()["items"] == []