fix: session count label distinguishes visible vs total

Show "X visible (Y total)" when the history list is filtered or limited,
and "X sessions total" only when every session is actually displayed.

Made-with: Cursor

backend/routes/sessions.js

@@ -67,7 +67,10 @@ router.get('/', (req, res) => {
       return { ...session, has_notes, notes_preview };
     });
 
+    const absoluteTotal = db.prepare('SELECT COUNT(*) as total FROM sessions').get();
+
     res.set('X-Total-Count', String(countRow.total));
+    res.set('X-Absolute-Total', String(absoluteTotal.total));
     res.json(result);
   } catch (error) {
     res.status(500).json({ error: error.message });