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.

crates/owlen-core/src/config.rs

@@ -708,6 +708,8 @@ pub struct UiSettings {
     pub show_onboarding: bool,
     #[serde(default = "UiSettings::default_input_max_rows")]
     pub input_max_rows: u16,
+    #[serde(default = "UiSettings::default_scrollback_lines")]
+    pub scrollback_lines: usize,
 }
 
 impl UiSettings {
@@ -738,6 +740,10 @@ impl UiSettings {
     const fn default_input_max_rows() -> u16 {
         5
     }
+
+    const fn default_scrollback_lines() -> usize {
+        2000
+    }
 }
 
 impl Default for UiSettings {
@@ -750,6 +756,7 @@ impl Default for UiSettings {
             wrap_column: Self::default_wrap_column(),
             show_onboarding: Self::default_show_onboarding(),
             input_max_rows: Self::default_input_max_rows(),
+            scrollback_lines: Self::default_scrollback_lines(),
         }
     }
 }