feat(commandline): full-history dropdown next to the input

Add a ▸/▾ toggle button on the right edge of the input row. Tapping
it pops up the full backlog of everything the command line has pushed
since startup — useful once the on-overlay rows have faded out.

The dropdown is a fixed-height scrollable panel rendered above the
input pill. `anchor_bottom` keeps the newest entry in view on open
and `opaque` blocks wheel events from bubbling through to the
viewport shader behind it (without it, scrolling the history zoomed
the drawing). Submitting a command closes the dropdown so the new
prompt is immediately visible.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/mod.rs

@@ -355,6 +355,8 @@ pub enum Message {
     CommandHistoryPrev,
     /// Recall next command in history (↓ arrow key).
     CommandHistoryNext,
+    /// Toggle the dropdown listing the full command-line history.
+    CommandHistoryToggle,
     ToggleLayers,
     LayerToggleVisible(usize),
     LayerToggleLock(usize),