feat: command history navigation with up/down arrow keys

CommandLine now records every submitted command in a recall list (up to
50 entries).  Pressing ↑ in the command input walks back through the
list, ↓ moves forward, and returning to the newest position restores
the draft text the user had typed before navigating.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/app/mod.rs

@@ -176,6 +176,10 @@ pub enum Message {
     CommandInput(String),
     CommandSubmit,
     Command(String),
+    /// Recall previous command in history (↑ arrow key).
+    CommandHistoryPrev,
+    /// Recall next command in history (↓ arrow key).
+    CommandHistoryNext,
     ToggleLayers,
     LayerToggleVisible(usize),
     LayerToggleLock(usize),