feat(ui): add configurable scrollback lines and new‑message alert badge

Introduce `ui.scrollback_lines` (default 2000) to cap the number of chat lines kept in memory, with `0` disabling trimming. Implement automatic trimming of older lines, maintain a scroll offset, and show a “↓ New messages (press G)” badge when new messages arrive off‑screen. Update core UI settings, TUI rendering, chat app state, migrations, documentation, and changelog to reflect the new feature.

docs/configuration.md

@@ -67,6 +67,9 @@ These settings customize the look and feel of the terminal interface.
 -   `input_max_rows` (integer, default: `5`)
     The maximum number of rows the input panel will expand to before it starts scrolling internally. Increase this value if you prefer to see more of long prompts while editing.
 
+-   `scrollback_lines` (integer, default: `2000`)
+    The maximum number of rendered lines the chat view keeps in memory. Set to `0` to disable trimming entirely if you prefer unlimited history.
+
 ## Storage Settings (`[storage]`)
 
 These settings control how conversations are saved and loaded.