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.

frontend/src/routes/+page.svelte

@@ -124,8 +124,11 @@
     seenIds = new Set();
     dismissed = new Set();
     history = [];
+    serverHistory = [];
     persistSession();
     P.saveJSON(BRIEF_VIEW_KEY, null);
+    // Signed in? Also clear the account (cross-device) history on the server.
+    if (auth.user) delJSON('/api/history').catch(() => {});
     showHistory = false;
     select(selected, true);
   }