feat(dashboard): infinite scrolling for the prompt list, saved default plus session toggle

- PromptManagerConfig.INFINITE_SCROLL (web_ui.infinite_scroll) is reported
  and saved through the settings API; only booleans are accepted
- Settings modal gains an "Infinite Scrolling" default; saving applies it
  at once
- the results header gains an "Infinite scroll" checkbox that overrides the
  default for the current session without saving
- when on, the next page is appended as the list nears its bottom (same
  load-more rule as the gallery), page controls hide, a short page keeps
  loading until the list fills, and a refresh after an edit restarts from
  the top

tests/test_api_admin.py

@@ -928,6 +928,29 @@ class TestDuplicateJobsAreSingleFlight(AdminAPITestCase):
 
 class TestSettingsMisc(AdminAPITestCase):
 
+    async def test_infinite_scroll_default_is_reported_saved_and_validated(self):
+        data = await (
+            await self.client.request("GET", "/prompt_manager/settings")
+        ).json()
+        self.assertIs(data["settings"]["infinite_scroll"], False)
+
+        resp = await self.client.request(
+            "POST", "/prompt_manager/settings", json={"infinite_scroll": True}
+        )
+        self.assertEqual(resp.status, 200)
+        self.assertTrue(PromptManagerConfig.INFINITE_SCROLL)
+        self.assertIs(self._read_config()["web_ui"]["infinite_scroll"], True)
+        data = await (
+            await self.client.request("GET", "/prompt_manager/settings")
+        ).json()
+        self.assertIs(data["settings"]["infinite_scroll"], True)
+
+        resp = await self.client.request(
+            "POST", "/prompt_manager/settings", json={"infinite_scroll": "on"}
+        )
+        self.assertEqual(resp.status, 400)
+        self.assertTrue(PromptManagerConfig.INFINITE_SCROLL)
+
     async def test_result_timeout_and_display_mode_saved(self):
         resp = await self.client.request(
             "POST",